Make sure you have the magic mouthwash/viscose lidocaine/whatever stuff the doc's have help with the pain itself. Swish with the topicals, then drink the shake when the pain killing effect is at its peak - immediately or after about 5 minutes, depending on the product.

The canned products use emulsifiers of some sort that make it kind of like ... latex paint. My husband found my concoctions more palitable. It will not be a complete meal replacement: my husband had Ensure at work, and my shakes at home. If the ensure/boost doesn't go well, try the following:

1 packet vanilla Carnation breakfast essentials
1/2 - 1 scoop whey powder (the natural vanilla kind)
1-2 cups milk

Blenderize it. Use more or less milk (or soy or almond milk)to find the best consistency for your dad. Vanilla ice cream be appealing. You can add Benefiber, canned peaches, baby oatmeal (or rice cereal), cooked oatmeal to the basic formula as he recovers. Canned peaches were the first fruit he could tolerate in the shakes - bananas were a bit more challenging, oddly enough. Everyone is a bit different.

When things were at their most wretched, my husband would drink his nutrition at the kitchen sink so if he coughed or choked it would be less messy. I think that it was easier for him to swallow standing up.

Best wishes with your strong-willed dad.