Another idea in addition to Rory Alsop's excellent answer, try goal-setting. Set goals for yourself, for the family and help him set realistic goals.
These must be tempered for younger children but the ideas are excellent. There are more ways to help in the link, this is their first point.
Confront unrealistic goals. Sometimes kids choose goals so big or so out of their element that it’s nearly impossible to meet them. When
my daughter first set her goal of becoming a champion, we had a long
talk about the difference between long-term and short-term goals.
If your cat-allergic child sets a goal of getting a cat, it’s time to
have a reality check. If your little basketball player identifies the
NBA as his goal, help him set a more attainable and age-appropriate
version of the goal for this year.
Encourage your kids to choose goals that are realistic. Whatever the
goal your child sets, be sure that your child came up with the goal.
If you want your child to follow through, the goal has to have meaning
to your child.
You are meant to make sure it is his goal, but at 3.5, this may not happen without you 'helping' with some gentle suggestions or guidance. So the goal at your child's age level might be something like: "I will learn to colour inside the lines this year." You help set up ways that
help to to accomplish this goal. You are modelling leadership, which is the easiest way to teach it.
Also, when you are doing an activity that your child already is comfortable doing -- put him in charge. "What do we need to do or have before we go to the park?" Allow him to make mistakes and allow him to say, "You need your car keys, and everyone needs a hat and sunscreen." If he forgets something important, ask about it in a way that will help him remember the next time. "I wonder if anyone will be thirsty at the park?" He then can reply, "We need our water bottles in the bag, too!" By being in charge and even by forgetting his shovel, he is learning to make choices and take on the leadership role.
I go on a lot about giving your child choices. This is vitally important to help your child make decisions. If you want more about choice, let me know and I'll add to this answer.
For teens -- here are some excellent ideas about setting goals I won't quote them because this doesn't answer the question.