Almost exactly my situation. Currently have my Class 2 medical only and use Steriod inhaled preventatives and ocassional salbutamol reliever.
Spoken to CAA about it and (following the submission of a doctors report) they were pleased to advise me that both the medication and asthma in general should pose no problem for Class 1 certification. It's a case of turning up on the day with your prescription etc, and doing the peak flows.. going running and then doing them again.
As long as the medication is inahled and not taken orally (assuming his asthma is not serious) then, going on what i have been told by the SRG, he shouldn't have any problems.
For more info.. I suggest you run a search for "Asthma". Plenty of info on this forum.
RC