Then you need to think carefully and honestly about whether yeh can accommodate the boy's needs, or what additional resources you'll need to be able to support the lad. Sometimes, if the conditions are serious enough and your level of medical knowledge is limited, yeh might have to say "we're not comfortable with your son participating unless we can get X, Y, and Z."
I think that's a great approach generally, especially for unit outings,but if this is summer camp being discussed, shouldn't the camp's medical officer at least be consulted, and possibly even make the final call?