Do you have access to dog clippers or know someone who does? It can be a good idea to keep the belly area clipped as it tends to get matted more easily and it can help keep them cooler. Since he is not comfortable laying on his back, I would not recommend using scissors. Clippers can be good to use to clean up his bum as well, but you do need to be extra careful to not cut anything delicate.
When you are brushing him, try holding under his arms and left him up so he is standing up on his back feet. This should allow you to brush out his belly with less stress to him.
Getting a blower is a good idea. Even if you aren't going to show, it really does help with grooming and keep more wool on the rabbit so is good if you want to use the wool. A shop vac with a blower port will work and is not too expensive (I got a small one for about $35). Blow out his coat instead of brushing and you should not need to brush as much. It can take a little while for them to get used to the noise and feeling, but they do get used to it. When you do blow the coat, hold the nozzle away from the coat, the air stream should open up the coat. If you are too close, the wool will funnel and that can cause matting. You should be able to see the skin and not have clumps in the wool. If you see clumps, then brush them out if you can. One warning, if blowing has not been done in a while, there will be lots of dander flying around. Try to do it outside or in an area that you are ok with getting dander and fur all over.
Also get him used to being handled and pet on his back. If you have only been doing it while grooming, he can be associating it with something he doesn't like. Try holding him on his back for a little bit as part of your regular routine. Don't do anything else, and then put him down. This helps him know that he is safe and nothing bad will happen.
