WELL-
If he's lived all his life with it then he should know his triggers
(Common triggers are exercise, dust and smoke, incense as well, animal hair, grass/flowers, cold & flus)
and be cautious of them at most- Asthma (Depending on severity) doesn't usually react to ALL of the triggers (for example, mine are colds and flu and exercise, my sister reacted to plants/flowers)
As far as I know, Asthma didn't have any treatment back then- so a good non-medication treatment would be standing or sitting in a place where the wind can hit your face, taking deep breaths- it usually takes 20 minutes to an hour before the wheezing fades
He should also avoid having anything too tight around his neck and I doubt he could wear armor at all if his trigger is exercise- (For short bursts it's possible, but long term definitely not-)
Also depending on severity- some asthmatics (*points to self V_V*) can wake up in a coughing fit or wheezing, finding it difficult to breathe- and have to sit up to make sure they can breathe properly, repeat back to 20 minutes - an hour-