This is what worked for me, once my taste buds "went bad" then went out, and carried me all the way through:

Whey protein powder mixed in milk to assure 1gm protein for every lb of pre-treatment body weight.

Cream of Wheat swimming in butter for breakfast when I could get it down.

Daily multivitamin/multimineral (I tried Boost for the vitamins but it was "oily" tasting to me and after a while I couldn't stand it told my wife to throw the rest out.)

Liquid L-Glutamine every morning during tx.

As much water or sports drinks as I could stomach (toward the end so little that the MO's office would have to "fill me up" with liquids even in between chemo sessions.)

If I could eat anything else (my problem was nausea, not mouth pain during tx) I did, but it wasn't ever much.

The RO said I was in the "top 5%" insofar as showing little adverse impact from the radiation, the tech people who administered the tx agreed, and gave the whey protein the credit.

Like I said, it worked for me, but check with your doctors before trying it. I stayed away from the nutritonists recommendation to "get calories any way you can," and lost about 30 pounds during treatment (from 210 down to 180.)I am stable now at 200 (21 months after tx ended).