My adult son’s weight

My son (22m) is 6’0” and up until 5 months ago he weighted 135 lbs so underweight then but, due to extreme stressors in his life he has lost more and down to 115 lbs. He has a slight intellectual disability and lives with me. His Dr knows about this and has just said to “eat more”. I’m at a loss. Most of what he eats, his body won’t let it stay. I’m worried about nutrition and weight. I’m hoping I can get some advice from someone here. We just started a few days ago but, I’m having him eat smaller meals throughout the day, I ordered Ensure for him that will be here Tuesday, stocked up on foods he likes and adding peanut butter and anything else I can think of. His stress levels are out of his control and will not lessen for awhile. Please let me know your opinions please.