lethargic fawn

My friend has a fawn that is one of triplets. He is about 6 weeks old. He is half the size of his two sisters. He just seems very sullen and lethargic. His eyes look droopy and not bright. He doesn't run and play with the other fawns and just kinda lays around. He seems to be eating OK, takes about 10 oz. in the morning and this seems to bring him down. The next feeding he might drink two or three. And he goes back and forth with the amount he drinks. It seems like the redcap slows him down, even all the others are doing fantastic. (His sisters even eat grain.) When they give him some gatorade, he perks up and gets some energy but later he crashes. He is giving nice pellets but when he was first born he had a little blood spots in his stool. it cleared up right away though. He just seems droopy and like I said is not near the size of his two siblings. I was wondering about giving him some B-12 or even some nutri-drench to try and boost him up. They do give him some fawn paste and probios but he doesn't eat much of it and it doesn't seem to help. I hate to see them lose him but he doesn't look good for the long run. You can tell there is SOMETHING wrong with him. Any tips for this little guy?