Cat vomiting (have already seen vet)

I have a 4 year old cat with my boyfriend. He’s strictly indoor and never goes outside. He is our only cat. Because my boyfriend and I have opposite work shifts, the car ALWAYS has someone there with him. He does not have issues with hair balls. Right now, he eats dry food only. He poops just normally, in normal amounts, each day.

He’s always had a sensitive stomach. When we first got him, he threw up a lot. We changed his diet and he was better. Then he began to vomit every day or every other day. He quickly lost a pound. We took him to the vet, who specializes in cats, and got fecal, urine, and blood tests done on him to rule out anything.

Everything came back negative. The vet had us put him on a special dry food that is specifically for cats with sensitive stomachs. He did fine for 2 weeks and gained weight again.

Now he is throwing up again. I want to try a different diet but the vet wants us to shell out $300+ for an ultrasound. I know he doesn’t have an obstruction because his appetite is very good, he poops normally, etc. Plus they already tested his fecal matter for other possible issues and it was all negative. The vet doesn’t care, still insists that we pay out of the nose.
I’m not an idiot. I know when I’m being taken for a ride and she’s doing that. I really feel that he has some food sensitivity but I’ve talked to 2 vets and both just want me to pay a ton so they can go up his butt or do an ultrasound.

What do I do?

If I can’t get this under control, I won’t be able to keep him because of the frequency of vomit and I’m in a tiny place with some carpet. He is a black kitty and I don’t want to take him back.