I completely agree - that's a good plan for you, but it's not appropriate for a 5 year old. If he just started playing T-Ball, it's going to take a while for his muscles to acclimate to the differences from his other activities.
Whatever you do, make it fun. I think the other moms gave a lot of good suggestions. Play kick ball, do "Red Light/Green Light" games, etc. If you make it anything less than fun, he'll likely learn that he hates exercising. But, if it's fun, it won't be looked upon that way.
Please don't push him too hard - he will resent you in the long run. You should encourage him to find something he really enjoys and do it. When I taught gymnastics, it was about making it fun for the kids so they wanted to do their best. I had to ask MANY parents to go to a different part of the building because they were trying to tell their kids what to do and not letting me be their coach.
Maybe T-Ball isn't his thing. Maybe he'd prefer something else. But, the current tactic will probably backfire in the long run. Who knows, maybe he's getting ready to go through a growth spurt and is cubbing up to have the stored calories needed for that.
Please also take a look at what he's eating. There are a lot of hidden calories in things we think are OK for our kids (trail mix is a prime example). Kids don't know how to overeat - it is something we teach them as adults. We let our kids have treats, but a lot of times, we give them a few options for snacks and try to keep them as healthy as possible (string cheese, fruit, popcorn, etc)