Are you supplementing the guinea pig's diet with vitamin C, and giving a large bunch of Timothy hay each day?
Are you seeing normal stools, and is there a runny nose, or a crust around the nose?
Let me know, disinfect the cage with vinegar and water, and then put him in a nother cage, a nice cage but with no drafts and with lot sof light. Make sure he is drinking and put a heating pad on low under one half of the cage.
Hope he feels better soon.