Trail mix was very popular with our kids, look up how to mix, but really just a mix of dried fruit & nuts, including coconut shavings. Good because you can snack on how much you want and easy to mix up.
Why not fruit? If he is a bit fruit-averse (you don't say how old he is) try cubes of fruit, either on a skewer, or the end of a fondue stick, that he can dip in various sauces. I know you want to avoid sugary things, but I used to find that the small amount of melted chocolate (thin it down with coconut oil) or maple syrup was a 'gateway' to eating the fruit.
And tortilla chips with dips like hummus or salsa are great.
Definitely search for your local wholefood co-op / shop or similar where you can find good stuff and people to advise.