Ideas to increase calories needed

My 5 year old has been tested with high cholesterol. We are in the process of testing, it probably is related to a thyroid issue, and in the meantime, we need diet changes. The problem: our diet is already low cholesterol friendly as a family, and my child is very active, prefers fruit over candy or other "junk" food, does not like fat, eats in general little, and as a result is also on the lower healthy weight for his age and height. I am supposed to further reduce his red meat, which is doable, eliminate butter and margarine, which is not an issue as I only cook with olive oil, eliminate completely processed food with hydrogenated fats which again will be easy, but also switch to low fat dairy. This last one is tricky because the kid is already avoiding fat in food like the plague, and 2 servings of dairy with full fat were so far helping with increasing a bit his fat and calories intake. What can I add to his diet instead? Need ideas. He hates avocados, so this one is out. When it comes to nuts, he only likes pistacchios and peanuts, but he will get bored if he eats them daily.