I've got triplets the same age as your son and they all do the same things, they eat at meal time and often crave snacks. Since I make sure that their meals are healthy and part of a complete daily plan of nutrition, I don't always worry if their snacks aren't simply veggies and fruits.
My triplets like to be very mobile while snacking, so I have these two-handled cups with a soft plastic lid that has cuts in it (like pie slices) that I give them with whatever snack I choose. Because I'm the only one around and I'm often doing household chores, I don't want to spend too much prep time getting these done, so I'll opt for any of the following "snacks" for the kids:
-Raisins
-Honeycombs (They're a great size & the honey has been cooked so no worries about baby botulism)
-Imitation Crab Meat (Chunk Style)
-Fig Newtons, each cookie cut into 4 pieces
-Goldfish Crackers
-Teddy Grahams
Sometimes I'll also just sit in my recliner and feed them pieces of chunk pineapple, peaches, pears, or mandarin oranges.
I don't really do veggies as a snack for them yet because I'd rather they ate them cooked at this point due to choking hazards, although black olives might be OK if they are cut up.
On the opposite end of this spectrum since he's only 21 months is that you let him cry it out and limit his snacks so he doesn't begin to depend on the snack as part of his food intake. You could try giving him a bigger meal or giving him a sippy cup with water or a water/juice blend or milk/kefir (but that's probably higher in calories which might defeat this purpose).
So far this has worked out great for me and my wife (on the weekends). Hopefully this will work out well for you too!
PS. I know the imitation crab meat may throw up some red flags, but it isn't bad for them. We checked with our ped first and my wife is a physicians assistant. Since it's imitation and not 100% crab meat, it's actually ok for them to eat and it's an interesting texture to introduce to them early on to help with eating new foods at later stages.