Well worst news.
Either diabeties or over active thyroid (quite unusual in such a young cat Socks is about 7) .... further tests would be needed to identify....... but as you correctly say Mammyshaz both are treatable .... however Socks is an independant cat who takes off for days on end so daily treatment would not an option even if affordable.
My daughter (23) in tears but also agrees he should not be confined .... he started life free on a farm, lived his life free to visit everyone and get fed by whoever he could talk into it... (he has a huge fan club) so he will have to take a final trip to the vet soon (but not yet!)
The saddest part is he will never get to go to Wales to live on our new smallholding which he would have loved.