Have him help you with things that involve holding, gripping, etc...like putting clothes in the dryer, picking up toys, etc. Make sure to take him to the park or back yard to SWING where he has to hold onto the rope or chain. Take small steps here.
If you find something he WANTS to do that involves gripping, etc., then he'll be more likely to want to do the exercises. Do not overlook these exercises. NOW is the time to start teaching priorities. Remind him that this is something he'll need every day for the rest of his life and you want to help him be as strong as you can. Make a game out of things, if you have to. BE CREATIVE. There are TONS of everyday little tasks that require grip.......NO! DO NOT JUST OVERLOOK THIS. Otherwise, you'll be creating a dependent child rather than an independent one. You don't want to be doing things for him at 8 or 10 that she should have learned at 5 or 6. Muscles have to be overloaded/fatigued in order for them to become stronger. It's amazing how quickly the body will respond when given the right stimulus AND you're persistent and consistent.
If he wants to watch a movie or play w/ something, remind him that his exercises are a priority and these exercises will help him every day for the rest of his life. He can watch a movie or play later.
It's one thing to be behind or struggle w/ skills, but it's another to allow it to create even more problems and or dependence down the road. Do everythig you can to help him become strong, confident and independent.