I didn't have a cavy savy vet, but the vets at our towns clinic were willing to look into them and their care and problems for me. I am very happy though, as a new vet came this year that IS cavy cavy, who will be doing my boys neutering later this month. maybe talk to the vets and see if they are willing to look into gps for you??/
Now, in the meantime, I just dealt with all this. My older male, Pumpkin, was 6 years and 3 months, but sadly passed away this past thursday. I noticed his weight going down about 6 months ago, and his over all body conition going down. The last months of his life(especially his last month), I was giving him unlimited hay (as you already should,) provided mushy pellets 24/7. Picked fresh grass for him, enough that it lasted over an hour, and lots of fresh veggies. I also fed him Critical Care 2 times a day the last week.
Now, I'm not trying to scare you with that Coco is going to die. I have another older girl, shes 5 and a bit, and she has gotten a bit skinner now. The vet says shes healthy, just aging.
So, until you can find a cavy vet, or one willing to become one, make sure you give him as much food as possible, and cater to him. With Pumpkin, he got soft pellets instead of hard ones, more food, etc.
Hope all turns out well and this is just Coco entering his senior years ( For Pumpkin and me, they were the best of our time together)
PS- an exotic vet gave me a recipe for Critical Care that they use for the guinea pigs at their clinic. If you want it, PM me and I would be willing to send it to you so you can make up some.