I hope the ultrasound uncovers the issue causing the biting and growling and that it can be treated so he can have some relief from that. I know it would be a relief for you, too. It must be so scary and frustrating and sad to watch your baby go through that for so long without even knowing why.
Have you tried some of the testing tips people give here to see if the testing would get any easier? It was hard when I first started testing Mia, but it got easier pretty quickly. I followed the suggestions of always doing it in the same spot and always giving her a treat and lots of praise and attention. I think part of it was also me getting more comfortable doing it. Now, she comes running and gets in her testing spot and holds her little ear out for me to poke it. I'm in and out quick, and she is on to eating her treat. Testing can save you on vet costs. You do not have to pay to go in for glucose tests or fructosamine tests.