Booster seat for 3 year old?

My son is 50lbs and 41inches tall. His forward facing seat now is so tight wear you buckle around his legs and the belt is extended all the way out. Like it literally cuts his legs.. he is in a cosco mighty fit 64. The only thing bigger is a booster right? I was looking for a high back booster seat but the only ones I found say only for 4 year olds but he fits the height and weight requirements. Well almost the height... he is 41 inches and it says for this seat 44inches.... Should I put him in a booster?