My favorite take-along toy has been a Moon Sand kit. It's definitely messy stuff, so you want to keep it outside, but it folds up into a small plastic "briefcase" with inflated edges & velcro to keep the sand in. It's not huge and it's not really a lap toy, but if he's short enough to stand (or maybe kneel) on the bleachers, he could use his seat as a countertop to play in his "sandbox."
You can find these sets at Toys R Us and sometimes Walmart/Target.
A less complicated take-along would be lacing shapes that use cardboard shapes or animals with holes punches along the edges. The child uses a shoelace and threads it up and down through the holes to outline his shape. If you're crafty, you could even make your own with cardboard and a holepunch.
Ooo--another idea. Does he need to practice his colors or numbers? You could make up a chart and have him count things (hats? shirts?) or put a checkmark next to each color he sees around him. Kind of like a super-simple scavenger hunt. It would require more prep work on your part, but it could be worth it. I actually conquered grocery shopping this way, by giving my kids a clipboard with a few pictures of things we were shopping for and putting them in charge of their "list." Just a thought.
If all else fails--take a couple of containers of tiny foods. Raisins, sunflower seeds, nuts, dried cranberries, goldfish. It helps if it's an unusual treat.
And if none of these appeal, try the Family Fun Magazine website for ideas. They're terrific and you may find printables or some activity that you can tailor to his age group.
Good luck to you. In some ways, it does get easier with experience!