Thanks for posting your query at DoctorSpring.com
I am Dr.R.K and I am pleased to assist you.
I went through your post about one of your family members losing weight despite having good appetite.
You have also mentioned that 'everything' comes out by bowel movement.
The possible causes are malabsorption syndrome and hyperthyroidism.
I would suggest the following tests - a complete blood count, thyroid function test, stool analysis, small bowel biopsy.
You can discuss with his gastroenterologist about the above mentioned possibilities and get all these tests done.
We can plan the treatment only after knowing the cause.
I hope that answers your query.
Let me know if I can assist you further.