Likely the chemo. It causes nausea, it's also very hard on the kidneys he should be taking in lots of fluids, constipation could be another reason. Also this early in the game he should try to eat to maintain swallowing. There is also the possibility that the issue is related to the formula - I couldnt tolerate any of them... I was by mouth the whole time. good in a way.

Try slowing it to a ver very slow drip rate and running the feeds at night maybe? Good luck. And do bring it to the attention of the dr. And or dietician.. Take care!