No Immune System.
That means at the slightest fever we will immediately start antibiotics to cover a wide range of possibilities. That is because with any infection he could quickly become septic, and the infection become fatal.
The skin on his bottom is also broken down - so we have to lather his bottom up with "triple-paste" to prevent worse rash or infections. That makes changing diapers quite unpleasant for him to say the least.
I've learned more than I ever wanted to know about cancer - for what it's worth my latest observations and unanswered questions are on our medical page.