just want to add that if anyone needs to have their seats checked by a professional, you can find one near you on
www.seatcheck.org
jodifla ~ I would check that out and see if they have any suggestions for your DS....I'm not a professional, so I don't know if this is safe or not, but perhaps you could put your DS in his booster, fasten the seatbelt then use a locking clip to hold the belt tightly so it won't come forward when he falls asleep.
My DD is in a Century Breverra booster seat and on the shoulder area, there are built in locking clips that I slide the seat belt through to hold it in place, that way it is snug against her chest and does not allow her to slump forward when sleeping.