Snacks for the picky no appetite kid

Maybe a bit of a rant too…. Are there any high calorie snacks that you would recommend that are easy to prep the night before? Trying to keep the weight on my kid because during the day on their meds they don’t eat and the school is beginning to have an issue with their current snacks because they’re processed foods. They’re recommending things like celery which obviously isn’t won’t help this kid maintain their weight. It’s so frustrating! Not all kids are the same. I just want mine to eat enough calories in a day at this point.