I don’t know about a MOLLE vest etc, I’ve not paid much attention but have seen kiddie ones in army surplus places etc, but these tend to be really aimed at little kids. Depending on his build he may be inbetween and too big for a kiddie one but too small for a small adult one
That may be a task to track something down, so perhaps find something a bit too big but modifiable and talk to either the ladies at a fabric shop or if you’re anywhere near a military town look for a tailors that does repairs and adjustments for soldiers
The Dye i4s will depend on his head shape. They are a small goggle for face coverage which tend to give good cover for kids. For someone like me there’s a lot of forehead and chin exposed in i4s
Then it’s down to the strap to how well that makes them fit around his head. New is better with more elasticity. If they just about fit but you’d prefer a bit more security then ‘double strap’ them by adding a second strap