Underweight Kid - Ideas?

Thanks for the ideas; there are some very useful ones here. A bit of a response - we have seen a nutrionist and she was quite helpful, but also suggested we give him ice cream after supper every day. That's not going to happen.

Have you tried fruit smoothies? My mom used to make them for me. Strawberry and banana and then whatever frozen/whole fruits we had in the house. Then frozen oj mix, yogurt, and milk, and then she'd smear yogurt on the inside of the glass I was going to drink out of. They're delicious. You can use whole milk and whole fat yogurt and play with how much of each you put in the smoothie.

Also if you have the funds for it, you might want to consider investing in a homemade icecream maker. It's an activity that's lots of fun and you can make it quite a bit healthier than store bought varieties. You can also do sorbet and such as well. (There are ways to make this stuff without a dedicated ice cream maker, but I don't know them and haven't tried them)

Edit: Forgot to add, stuffed chicken makes for a great meal that I know I personally adore. It's very calorically dense what with all the cheese and such. You can also shred it and add it to the tortillas. Also have you tried meatballs? Homemade sloppy joes? Steak sandwiches? All of these things can be done such that they are both nutritious and calorie dense (Alright I'll admit, sloppy joes would take work to be considered nutritious but darn it they're tasty!!!)