TAKE HIM TO A VET! Excuse the big writing but it's really important that you get him to a vet...Anyway, luckily I know a lot about guinea pigs. 8 years experience
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I have 3 guinea pigs. One of mine was like that and I didn't take her to a vet soon enough and she died. Well she needed an operation anyway...
If you have already taken him to a vet, make sure you give him a nice comfortable place to stay, away from your other guinea pig (you don't want 2 sick guinea pigs).
Since he's losing weight, try giving him nice soft food. Try Apple Puree and grated carrot. Since he has diorreha, make sure he has access to fresh water at ALL TIMES. He'll be losing fluids and if he can't drink from a bottle, put drop by drop on your finger and feed him that way. also, avoid all lettuce, it causes diorreha in guinea pigs. Never give him iceberg lettuce, even when he's well.
Keep him with you, even at night if possible, and definitely keep him inside.
If the worst happens... and he dies
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... make sure you get a new companion for Ant. Remember that if you get a new guinea pig, quarantine it from the others for at least a week.
Hope that Dec gets better soon!
StarTama