Sounds delicious! I'm not sure how the sour cream would hold up for baking, but he could always put it on the side.
Nut butters are also quite carby and calorie dense. You can make actual brownie/muffin things with pureed squash/pumpkin (or maybe banana) and nut butter, eggs, etc. Throw in some protein powder to up the protein. Could be quite the nutrition bomb.