I absolutely believe you should force activities on your young child, for the reason you found with the Taekwondo: Without trying it, how can he tell if he likes it?
As adults we can recognize when we don't like something because we've tried it -- or something similar -- before, vs. when we are uncomfortable just because it's something new. If it's the latter, we can force ourselves to do it anyway, hoping we'll enjoy when we've gotten used to it. But a five-year old has no such experience, or the self-control to act on it if he had. It's up to the parents of such a child to lead him safely into exploring the various activities available for kids. You don't want your child to miss opportunities for fun, for self-esteem, for a life-long addiction to healthy exercise just because he has a natural inclination to hang back from new activities.
There are pitfalls, however, and it's great that you're concerned about them. You definitely don't want to push him into something he doesn't want to do, or force him away from his natural interests, but the fact that you're worried about it means you probably won't be one of those overbearing parents, trying to live their dreams through their kids.
So, tell him that nobody knows what they will like until they try it, and you want him to know what things in life will be fun for him, so you're signing him up for one or two more new things. If after the lessons are over he still doesn't like it, he won't have to do it again.
But, consider the following carefully, before you push ahead:
Is the activity age appropriate? Soccer at this young age is about having fun. The coaches should be fun-loving and enthusiastic and the kids should be smiling and laughing. At the end of each game, all of the kids should be convinced they were terrific, and the coaches should only have a vague idea (if any) of the score.
Does he find it energizing to run around with other kids, or draining? Kids who are energized by playing a team sport can probably do more than one team sport per season. Kids who are drained by it should not play more than one team sport per season, and they may need some alone time or parent time afterwards to recuperate.
Does he object to the sport or to learning it from you? Some kids seem to get into an antagonistic relationship with a parent when the parent is trying to show them something. They take direction/constructive criticism much better from an official coach or teacher, and want to hear only praise from their parents. If this is the case, don't force your kid to learn the activity from you, but sign him up for something taught by others.
Does he only enjoy things if he is good at them? Some kids can be the worst kid on the team but still have as much fun as the best kid on the team. Others need to see that they can hold their own with those around them, and are truly miserable if most kids are better than they are. Figure out which type yours is and take it into account when you look for things to sign him up for.
Can he pick it up later anyway, and be no worse off then if he starts late? Bike riding is something that does not need years of practice; unlike in soccer he won't be hopelessly behind his friends if he doesn't start riding a bike until he is eight or ten. And at some point he will probably want to do this one, anyway, because it will give him some freedom and his peers will be doing it.
Is there another path that can lead him into the activity? Baseball, like soccer, requires skill and a lot of practice, but unlike in soccer, kids are generally a mess at it until at least seven or eight, no matter how much they practice. If he doesn't want to play catch with you, and you decide to hold off on signing him up for T-ball this year, you might try watching MLB games with him, rooting for your team. We absorb so much about baseball by osmosis just by growing up in America that we forget that the rules of baseball -- from just what is a strike, and how many you get before you're out -- and what does "being out" mean, anyway? -- to the infield fly rule -- are amazingly complicated. If he's watched some games with you, then if he does start playing later, he'll at least know which direction to run the bases. (Not self-evident to most five year-olds.)
Is it a survival skill? Most parents I know insist that their kids take swimming lessons. (I motivated my extremely reluctant daughter by promising her the night before that if she learned to swim I would let her take surfing lessons. She then spent the next two weeks throwing herself into the water -- she had just seen Lilo & Stitch and loved it -- and was the only four-year old to pass out of Level 1 at the end of the first session.)
Are you paying attention to the things he's interested in? (I do see that you are, but I'm trying to make my list more generally useful to others.) If you as parent are interested in sports, make yourself think of the other things that are out there (like ukeleles!). They also have dance, music, and art classes for kids in most communities. There may be a Lego club at the library. The point is for your child to be happy, learning and mastering new and rewarding skills.
Don't over-schedule. Kids need unstructured down time, to wander aimlessly, to develop an internal life, and to learn how to entertain themselves when nothing is going on.
After 2 - 3 of years of this, he will have tried many different types of things and he -- and you -- will have a better idea of the type of thing he likes. Fast sports or slow sports, team sports or individual. Dance, music, or art. At this point you should let him have complete say as far as what activities to be involved in. (Not counting things your family or culture insists on, for which you give him no choice, like... swimming lessons; Hebrew or Chinese lessons; a musical instrument; at least one sport, for the health benefits.) You can urge him to change his mind, if you feel strongly about something, but he gets to choose.
As far as motivation: I generally tell my daughter the truth: I want her to try things so she knows what she will like. I am against bribery, but I do believe in "celebrating." When my daughter spent what I thought of as one too many skating lessons pushing a chair around, I mentioned that the first time she skated a whole lesson without the chair, the two of us would go out for ice cream to celebrate. Needless to say, we had our celebration -- of her acquiring this amazing skill of skating up and down the ice all by herself!!! -- after the very next lesson.