It must be very frustrating for a boy that age to be "trapped" like that with his medical issues and I can see that he'd think college would seem like an eternity at 13. Do they have a social worker or counselor where he goes for his treatments? If I have a question about my daughter I usually start with her OT since she's gotten to know her well and is interested in helping with the whole package, including the self esteem.
Before I had a child I remember listening to older people saying, "what are all these kids so depressed about, they have so much...why are they so unhappy..." and seeing Oprah shows with teens talking about how depressed or stressed they were when the world seemed layed out at their feet and it was hard to understand.
But now with a child in school and really hearing so many stories, I don't know what the reason is but plenty of kids are really and truely depressed and stressed. A boy classmate of my daughter's said he was going to "pass away tomorrow" and my daughter asked him what he meant, and he said he was going to jump off a building. This is an 8 year old. When you mention it to the teacher though they seem to think it's probably just some sort of joke or teasing. Maybe some teachers take these things more seriously than others. Same with parents of kids that age if you try to tell them. I don't know if it's part the child being upset about something in their life at the moment and part them hearing about suicide from the news, and putting it together and not knowing what it really means or not, but it's better to err on taking it seriously whether taking immediate action or setting up appointments and letting your child see clearly that you're in their corner and are trying.
