I have no experience of this but when my daughter went to uni they had to register with a uni G.P. If your son has had to do this are you able to write discretely to the G.P. and outline your sons history? Does he look as though the weight is still dropping or is it disguised well. Maybe stepping back is the right thing but as his mum it is so hard to do. Obviously he knows how concerned you are but you may have to tell him that whilst you are really worried about him you love him trust him to look after himself and do the right thing health wise. Sending positive thoughts and good wishes. Good luck. Your son is lucky to have you love and care about him.