Peach Shake:
5 slices canned peaches (get the kind with only water, sugar and peaches - Del Monte Lite makes some)
1 cup french vanilla ice cream
1-2 cups milk (dairy, soy or rice)
1 packet Carnation Breakfast Essentials, Vanilla

This was what got us through the worst of the worst.

As he got better, I added:
More peaches
Benefiber
Whey powder (with natural vanilla)

Oatmeal Shake:

I got this idea from a posting on this site. Here is my version:

1 cup prepared Oatmeal: (I get Quaker 1 minute oatmeal, and simmer 2 cups of it with 4 cups of milk for about 10 minutes. This makes it less scratchy.)
2 small or 1 large VERY ripe bananas (or other fruit)
1 Packet Carnation Breakfast Essentials (vanilla)
1-2 cups milk (dairy, or mix of dairy and rice, soy, or almond)
Honey (if the bananas weren't all that ripe)

Note - both of the above also taste good to people that haven't had therapy - so you could make and share it with him. Make it thin at first, and be careful with amount of fruit because of the acidity. Good luck!