high back booster

I have a question I feel kind of dumb for asking but oh well here it goes. My son is 5 he's a big guy (not fat just big) but he is currently in a high back booster. Donny question is with a belted high back booster do you just belt them like you would yourself or should you lock out the seat belt after they are buckled? My husband wants him to be extra safe so he locks the buckle out so that way he knows he ain't going no where. But I'm worried that may be wrong or may be detrimental to his safety if we were to get into an accident. I'm still new at this mommin thing obviously he's my eldest and this is my first encounter with this. I've tried online research but it doesn't say much about locking out the seat belt. But if you all know please let me know I just want my little boy the safest he can be!