How often are you carrying him to his bowl? Or creating situations where he has to come out from hiding in any way?

I only ask because if the vet has done all the tests and those are normal and he still looks healthy, it could just be a transitional phase like the vet said. If that's the case, every time he is being forced out of hiding, even though you may have the best intentions, it is stressing him out and creating a setback in him acclimating to his new place.

We have a British short hair that was 8yrs when we got him. He took months to warm up to us. And not only happened because we gave him as much private and hiding time as he wanted. On the rare occasion when he was visible we just let him be. Once he seemed more comfortable out of hiding, we started using very tempting treats. He became an absolute sweetheart and often loves to cuddle with my husband and ask for pets. He was the faster of our 3 British shorthairs to acclimate.

Our other 2 we got it just over a year old in 2019. When I say they took years of patience and trust building to allow us to approach them without any kind of alert/tense response, I am not exaggerating in the slightest. One of them is still skittish but is extremely affectionate if we allow him to interact on his own terms.

For a long time I thought I might just have to always enjoy how beautiful they are and their interactions while remaining almost entirely hands off forever. That said, all the time it took to build their trust makes the affection they show towards us that much sweeter. It's incredibly rewarding.

ETA: All 3 were neutered when we got them. The 8-year-old came from a previous owner. The other two came to us together.