the muscle stiffness could be brought on by something else entirely -- a heart problem, perhaps?
If he's used these drugs for years without harm then it's likely they're not the culprit. You might, to be on the safe side, want to have his asthma checked out, however; muscle cramps can be caused by a lack of oxygen to muscles. If his breathing is off, his muscles may feel it and cramp. this could be a sign it's getting worse.