I'm going to dissent here. I think what you describe - while perhaps light on protein - seems pretty healthy. I think as long as he's at a good weight, just feed him until he's full. Yes, up portion sizes at dinner but what's wrong with an after dinner snack - a banana is healthy and filling and crackers may be empty calories but for a growing / hungry boy it might fill him up. It could be he has a fast metabolism so he fills up at lunch/dinner but just burns through it all in 2 hrs or so, but he is getting the necessary nutrients from meals so I don't see why he also needs all his snacks to be densely nutritionally packed the way you would need to with an underweight kid/toddler. Great if you do have a second dinner to feed him but I wouldn't stress over it - otherwise let him snack. For snacks, just keep anything out of the house that you don't want him snacking on (cookies, candy, soda, etc) and stock up on snacks you think are healthy (maybe deli meat / meatballs / hummus / frozen pizza???) and let him go at it.

I with everyone- more protein. You could do healthy protein like bean burritos or tacos. These are so easy to do from the can with a little spice and tortilla warmed in the microwave. It would make a substantial snack or simple protein dinner with sides.
Non fat Greek yogurt (flavored) with a dollop of peanut butter and honey is a big hit. I provide a side of fruit for fiber and vitamins.
Hard boiled eggs, walnuts, cheese, and a healthy sandwich meat are good sources of protein.
Active kids get hungry! My seven year old skinny mini boy already eats more than I do! If kids do things like swim on a team (mine does), fill the fridge. They will be starving the moment they exit the pool.