We have a scout with these exact issues.
He does just fine, and here is how.
Asthma
1. He has limit to just how much physical exersion he can do in a given time period. When he hits that point he stops and rests.
2. He always has his inhaler
3. The ASM medic always has a backup inhaler
4. The Senior scouts and leaders have all been taught what to look for, and how to treat him should he have an attack.
5. He always brings along a battery operated Nebulizer and can self adminster or ASM assisted a breathing treatment.
Allergies.
1. Stay on trails as much as possible when moving about.
2. Sets his tent and camping area as far from what he is most allergic to in the area.
3. Pants all year (Hell in the Texas summer)
4. Long sleves anytime they go out an about.
5. Alergy pill on schedule like clockwork.
This has been a trial and error development over the past couple of years with him, and we pretty much have it down to a science.
Best advise start him out slow, with a couple of day outings. An hour or two at first. Monitor him as he goes allong, and pull the plug at the first sign of trouble. then move up from there till you reach his current limitation.