I'm not a doctor so I probably won't be much help, I thought i'd input though!
I found a lot of medications that I, personally have been on can cause weight loss and nausea - which really takes away your desire to eat. Maybe the medication he is on is causing him nausea or contributing to him not wanting much to eat. In turn making him feel quite sick. I threw up a lot on my medication and a long time after I had no appetite, and when I ate certain things I felt faint and vomited. Just a thought. Or it might be something else entirely. Take care!
i am so sorry to hear that
have your son ever taken Culturelle Probiotic