Weight Loss Losing weight

I'm a little stumped and wondering if anyone has an suggestions. So my newest pig, Max, has the neurological symptoms. As mentioned in a previous post our cavy savvy vet is puzzled by him and we really have no idea what's wrong with him. Best guess is possibly a bacterial meningitis, so he's on high doses of Bactrim and Baytril. His head bobbing does seem to be getting better, he still does it but maybe not quite as much or as bad. My problem now is that he keeps losing weight. He was quite underweight when we got him and the vet said appeared to be malnourished. Since getting him, he's been eating unlimited hay, Oxbow pellets, and gets veggies twice daily (red and green leaf lettuces, peppers, carrots, tomatoes, green beans, grapes, blackberries, parsley, and cucumber - obviously not all at the same time and very limited grapes and blackberries, but mixed around, this is what he and my other 5 pigs eat). He eats like a pig, literally, but is still losing weight. On 12/4 he was 740 gms, 12/8 725 gms, and today I just weighed him at 677 gms. Should I give him more veggies? As is, he gets unlimited hay and fairly unlimited pellets (his bowl is never empty, but he does eat them, and I've been giving him more than my other guys, but he seems to eat about the same amount). Any suggestions would be GREATLY appreciated!