Baby weight gain

My sons 11 1/2 months old and had become pretty active. His weight has been bouncing between 18-19 lbs for a little while now and I keep seeing in all his papers from the doctors how he has "acute severe malnutrition but upon assessment looks well nourished" (exact words from his visit on Friday but they all basically say the same). This is 100% based on his weight.

He's 100% tube fed formula and gets 36-42 oz in a 24 hour period (which they said is fine because of his poor weight gain). He eats very little by mouth.

I just need tips on how to help this kid gain some weight.