Yes, the belt is useful. It's intended purpose is to keep the torso rigid and fully supported during any heavy lift, in laymen's terms. A good rule is to use a belt anytime the weight on the bar is > BW, or 60% BW for an overhead strict press. Your boy is ready for one.
I use a 3"
bestbelts but suggest seeing if your boy can somehow trial a few different sizes before purchasing. The height is important as the belt should fit between the bottom of the ribcage and the hip. You don't want to spend $100 only to find out it doesn't fit properly.