I knew a girl who suffered from cystic fibrosis - she attended the same dance school as my sister and I. Also, there was a young man who trained at the uni who had cf, he didn't train long, but held a black belt in aikido.
Presumably if this lad has been cleared to proceed by the doctor, and the parents know what is involved and want him to attend.
At the end of the day, it is only the individual who knows their limits, and when to stop, whether we are suffering with an illness, an injury, 100% fit, or totally hungover. Provided a class is reminded periodically that you are not a psychic, and it is up to them to stop if in any doubt.
In your place I'd try not to single him out in class, but discuss in advance if he may wish to be treated differently, e.g. before a long and physically tiring grading, discuss whether he would like an alternative route - solo kata, bunkai, teach an ohyio, take a written exam, ...