Sick What could be wrong with my Guinea Pig?

I have a 2 year old pig named Remmy, I've had him since he was about 3-4 weeks old and he's never had any health issues so far. He is a bit more skiddish than his cage mate and doesn't like to be handled as much. He does love to be scratched on the head and chin but that's as fR as he will let you go with him. There are times of course where handling him is inevitable such as when I have to clip his nails, change his bedding, when bath time comes around and just to do an overall checkup on him.

Recently I noticed that he looks a little more frail than normal and when I went to pick him up to check his weight he gave no resistance whatsoever to me picking him up. He didn't run away or squeal or anything, this is not like him at all. He does eat and drink just fine so I know it's not lack of food or water. I make sure that he and his cage buddy get unlimited timothy hay and fresh water as well as their recommended amount of pellets and veggies. I make sure to not give them to much veggies that have high calcium and I make sure they get enough that are good with vitamin C. He has lost weight and he feels really bony like he is very malnourished but like I said he eats and is fed well, and he poops and urinates fine too. I'm going to take him to the vet on Wednesday, but I was wondering if anyone knew what could possibly be wrong with him.