[souffle]

A vet visit is definitely needed to see what is going on here. Does he climb at all? It sounds more like he has fallen and damaged something either in the leg itself or in his back. Sometimes internal swelling can press on the nerves making matters worse. The hip could be dislocated. This might be why there is swelling on his abdomen.
7 months is too young for arthritis. Occasionally they may get it but not until 18months plus I would say.
Hopefully the vet will be able to find the cause. Some anti-inflammatory meds and painkillers will help but I'd advise cage rest in a smaller cage with climbing opportunities and flat surface and no wheel. Give plenty alternative enrichment like chewing sticks, cardboard to trash and sheets of kitchen roll as well a lot of attention. Let us know how he gets on at the vets and good luck