I have Asthma and I take Toprol XL. I was on Inderal at first and it did make my breathing a little more hard. Beta blockers have a tendency to do that, but they are also great at keeping down blood pressure. I take Singulair and Advair (plus an Albuterol inhaler) for my asthma. It seems to be under control even though I take Toprol XL. Toprol is a good medication so it may be worth it to try it and see how he does.