Sick Young guinea pig acting very lethargic and not eating much

My male guinea pig who is about 9 months and lives with my other male who is about 4 months has been very lethargic lately, just wanting to hide. He was eating and drinking normally up until yesterday. He seemed like he was interested in the fresh veggies I offered but didn't eat them last night. After not seeing him eat or drink for a few hours, I began syringe feeding him vitamin c, water, and food every few hours for about the past 12 hours. He willing took everything I offered from the syringe. His nose, eyes, ears, boy parts, and bum is clean and clear. He hasn't shown any signs aside from what I mentioned.

If he had a viral or bacterial infection, I'm surprised it didn't spread to the younger boy who is much smaller than him as they live together and are always touching noses and what not. He's as healthy as ever, eating and drinking normally, popcorning about etc. I've considered the possibility of my sick boy maybe being depressed since I did have another girl piggie living under this roof who was around most of his life. Although they didn't live together, they knew each other and would chirp to each other and whatnot. She went to live with a friend of mine and other girl piggies this weekend.

Money is very, very tight and he was taken to the vet a month ago and given a clean bill of health so I'm hesitant to call the vet. If anyone has any idea what could be going on, please let me know! I've just been feeding him mushed oxbow pellets mixed with warm water and vitamin c.