It depends what he normally eats. I'd take something like: a small sandwich (one slice of bread), cut into three with cheese or jam, a banana (don't peel it, mash/ cut it when you're there), a pot of raisins, a yogurt or baby fruit pot.
My 10mo DS loves pasta salad - some cold pasta with soft cheese, pesto and some spinach. Sometimes I chop up some mushrooms or sweet peppers, whatever is in the fridge. He can eat it with his hands and it's not too messy.
Rice salad is good too.
Or as others have said, sandwiches, yoghurts, fruit puree pots.
My ds adores blueberries and they are fantastically portable, just wash and go!
Actually he has no teeth yet! So he will have a go at a sandwich but he will struggle. I worry he doesn't get enough if I give him a sandwich if you know what I mean. Still I could take that and a bunch of finger foods to back it up.
Not tried him with blueberries yet - what a good idea!