Soy powder milk for infants works as well (I think that’s what your describing?). Are you able to get meat baby food? Or make scrambled eggs? Because while I’ve found that ensure makes my girls gain weight fast, those other foods do a pretty good job as well. Otherwise, if he’s eating well but still not gaining weight, it seems probably (to me) that something is still going wrong internally. I’d consider taking him back to the vet to make sure everything’s still going well (also check for dehydration, because a dehydrated rat will act lethargic and have little appetite - and dehydration is serious in rats so consider hand feeding with a needle-less syringe to make sure he gets enough fluid). I know I discussed my Latte before, but I really think hand-feeding might be something you should consider. My Latte has a pituary tumor and physically can’t hold food or even chew hard solids well - so hand feeding her twice a day with liquify foods keeps her both hydrated and nutrient filled (plus all my girls all LOVE eating from a syringe for some reason so getting her to take food this way is actually pretty easy).

 

If he's eating and not gaining weight the worms are probably still active in his tummy. Until the worms are treated keep feeding him foods high in protein and fats. Eggs work well, pasta not so (mostly carbs). Dip a little bread in olive oil, it's very high in EFA's. Actually meat with the fatty parts around it (chicken, pork, beef, it doesn't really matter). Brake up the bones (chicken bones are easy if boiled) and extract the marrow. Rats lap it up and it's super-rich in fats and protein. Basically you're trying to balance the dietary loss to the worms and keep the rat even-keeled.

 

I'm not sure about an alternative to Ensure, though