For a child who is potty trained, or at least is physically ready for potty training (often happens around two, but it varies significantly), this is approachable in a very simple way: ask him to use the toilet prior to the bath.
At some point, often around potty training, this will become voluntary for many chlidren - like our three year old - who is very nearly frightened of urinating in the bath now. He goes every time and will even leave the bath if he thinks he has to pee.
But for those children who are not cleanliness-obsessed, it can simply be made a requirement pre-bath: urinate and you get your bath toys, urinate and we'll put bubbles in the bath, whatever works. Of course, you can also just not worry about it - the urine is not particularly harmful and will not be in sufficient concentration to smell badly.
Physical readiness for potty training is primarily awareness of the need to pee, awareness of how to do so on command, and ability to hold it in. Once these three elements are present, a child can be fairly easily potty trained, assuming he or she is willing. (Pee and poop are basically separate in this regards, both have the same basic elements though.)
If he's goofing around peeing on purpose in the bath, that may well be an excellent way to start potty training - because it means that he at least is aware of how to do so. I would use it as a transitional device.
First, the next time he pees in the bath, congratulate him. "Wow, nice job, you know how to pee! You're almost ready for potty training!" Then, after a few days of this, instead of going to the bath, go to the potty, and let him know that he can do that on the potty, too! Make a game out of it. Rewards are great for this - M&Ms, for example; stickers; coloring sheets; or in our case, we managed to get in an online garage sale a bag of old Matchbox-type cars - something like 100 for $5. Enough that we used them for potty training our first child, and now have plenty left for our second! I would give him that reward once for going in the bath, and then tell him that if he does it on the potty he can have more rewards. (Yes, I sound like a drug dealer... sigh.)
He may resist doing it on the toilet at first, in which case don't push too hard - but remind him of the reward if he does. When he pees in the bath each time, remind him he can pee on the potty for a reward. Eventually this can become a very effective way to get him into the habit of peeing on the potty.