The first thing to check is if the seat straps are truly tight. Obviously, you don't want to totally squash him in there; however, if your whole hand fits between his body and the straps, they need to be tighter. A rule of thumb for confirming strap tightness is if you can slip two fingers (index and middle) between his body and the straps. If you can easily do that, make the straps tighter, if you can't do that at all, make them a bit looser. When you are tightening the straps, always make sure he is fully leaned back in his seat.
Along the lines of ensuring his straps are tight enough, be sure whatever he is wearing is what he will have on for the duration of the trip. If he has clothing - such as a large coat - that he removes during the trip, be sure and adjust the straps afterward.
If his straps are indeed sufficiently tight; your next step will be to simply find a way to adjust his position. Use a neck pillow (one of the ones mentioned in another answer) or just a super thin one. If you would prefer not to use any type of pillow, a folded towel or blanket may be used. You may need to experiment with positioning a bit.
If his straps are loose, that is indeed a safety hazard. Should you be involved in a car accident he could be ejected from the seat. However, if he has just found a way to be a bit "slippery in his seat" or beat the system, I wouldn't worry about it. If you were to be involved in an accident with him in that position with properly tightened straps, he will have the same injuries whether he was sitting bolt-upright or not.
Above all, if nothing else works, keep in mind that children are super flexible and the chances of him ending with any form of long-term injury are very low. If you are still concerned, you can contact your local fire or health department and see if they will do a car seat inspection for you. Some states here in the US require all fire departments to have at least one certified car seat inspector on staff during regular business hours. Typically, this will be a free inspection; however, if there is a fee associated with it, expect it to be very low.