Sorry for the delayed response! I have been on vacation and just got back in today.
I am biased due to my son having enzyme issues but they usually start around 6 months old symptom-wise. My DS Danny will be 4 years old 7/13 and has exocrine pancreatic insufficiency among many other complications/issues. (The root is still being determined for him.) He was 18 pounds at 6 months old and then stopped growing all-together. He weighed 18 pounds or less for a full 18 months. He finally started a high calorie formula (Neocate Jr.) and gained 2 pounds in 2 weeks. Then he more or less stalled again. He weighed 21 pounds 3 months after his 2nd birthday. He started digestive enzyme supplements to replace the enzymes he didn't make and he gained 8 pounds in 4 months. Since then however he has fluctuated between 30 and 31 pounds... This is due to some other reason which we have not yet found...but if your LO continues to lose or simply fails to gain for an extended time period I would mention this to your docs so that they can check for this. Our doc did the enzyme supplements before the actual procedure to see if they helped him. He gained the first 2 pounds in 30 days.

I really hope he has just had an off week and will get back to growing soon, but if not we will be here for support or advice where ever we can give it.
Suggestions...
You can try upping the calories per ounce in his formula by mixing it thicker. Danny was on 30 cal/ounce with the neocate jr. We still added more formula to the water at some points and eventually added canola oil to his formula to raise the fat and calorie count when it stopped helping him. Keep us updated on your LO. This board is slow but I try to check it every time I get online.