Son with autism has unexplained weight loss

My 45 year old severely autistic son for the second time in the last 4 years is having extreme weight loss. He is nonverbal and does not express himself in positive ways. Fours ago in a period of about 3 months he lost about half his body weight. At the time he weighed about 190 lbs. He dropped to 97 lbsduring that timeframe. About a year ago he started gaining some weight back. He got to 150 lbs. Approximately 2 months ago he started losing weight again. As of this past Thursday his weight is down to 120 lbs.

I have had him to numerous doctors for tests. ie. colonoscopy, upper endoscopy, CT scans, blood work, etc...

He does have an issue with very low iron and has had to transfused numerous times over the past few years.

Oh yeh! He eats like a horse! Although lately he is becoming more picky about the food he is given. He drinks a lot of water and milk.

As of late, a nutrionist has put him on a high protein, high caloric diet. So far there is no change other than the continued loss of weight.

I am at a total loss here in Southwest Michigan!

Thank you for any ideas!!!!