Personally, I would find a better vet. He needs his pelvis and chest xrayed - as soon as possible. There is no point in guesswork - if the vet is wrong in his 'guess' it could mean the life of your cat.
If his pelvis is broken, the most important factor is whether he can pass urine & faeces normally, as the nerves affecting these come off the spinal cord in the pelvic area. If there is no nerve damage and he can go to toilet, he needs strict cage rest for a few weeks. The fractured section of his pelvis will always be weaker, so I would seroiusly consider keeping him indoors in the future.