I put a rubber d-ring snaffle on my horses bridle the other day. She obviously doesn't like it.
For many horses rubber is very dry, which causes friction and discomfort. They are also often overly thick. I tried using rubber bits starting colts when I was younger, but quickly realized that they were much happier going directly into a much thinner metal snaffle.
Finding the right bit is trial and error. I generally find that many horses do fine in a stainless single jointed snaffle, but a good portion of horses prefer a double jointed bit, and many would prefer sweet iron or a copper inlay over stainless. My arab loves her single jointed stainless bit, and hates all others, while my appy loves a sweet iron mouth piece, and hates a double jointed bit. Its all trial and error.
Before trying different bits, get his teeth done, and if he is bad with grabbing the shanks, maybe try something with an s shaped shank, or a really swept back one, although I think a snaffle would be ideal.