I would feel on with the HBB, but first I would check to make sure he can buckle himself in the HBB. For my DD2, it depends on the car. She can buckle herself in our Honda Pilot, but not in DH's BMW, nor my moms VW. Even I have a hard time getting her strapped in the other cars because of the way the seatbelt latch is. I think it may be easier for you to buckle a harness than to reach over a HBB to latch a seat belt.
Sorry- I'm sure this is difficult for you!