13 Year old needs to gain weight - Help!

My son (13) is underweight. He has always been a tall stringy thing. His doctor wasn't ever super concerned because his growth rate was tracking okay and he was eating, just not a ton.

Well, now she's getting worried. He is tall, about 5"6 now I believe and he weighs (at last check in) 98lbs. My son is also getting worried for his own health.

I'm looking for suggestions on how to help him gain weight. He does not like cheese (that's always the first thing the doctor suggests) so that's not an option. He eats and enjoys a lot of very basic plain foods like noodles with butter, plain sandwiches, etc. He loves avacados so I have been buying those for him to munch on.

Are there any powders or drinks or something that can help him gain weight, but that also taste good to a kid? We tried ensure but he said they taste funky. He likes the carnation breakfast essentials but I'm looking for something with more calories.

Any help would be much appreciated!

ETA: Sugar is NOT an issue. In fact his, at last visit, his doctor said "I don't usually condone extra sugar, but in your case, you can have as many milkshakes as you want".