I am sorry to hear about your darling rescue cat having trouble with his hips… I know exactly how you feel, wanting to make sure you have exhausted every avenue before making a very difficult decision…
I would definitely encourage you to have an open conversation with your vet about how you are feeling… I think getting some x-rays of his hips and possibly other areas (eg. spine), depending on his clinical signs/pain, would be a good first step. Once it has been established that his pain is due to osteoarthritis in his hips (and not another cause) then certainly medical management is one option, but you could always ask whether referral to a specialist small animal surgeon would be an option for him. It is important to know the entire medical picture of your darling to know the risks/benefits of different options, so definitely chat to your lovely vet about this. There may be reasons unknown to me that make this option not advisable.
I hope this has helped you going forwards with your little mate. Let us know how you get on with your vet. I am sure they will be more than happy to chat to you about all of your options.
Best wishes and Merry Christmas!