Great question.  Actually I think you're very creative to think of this method.  We use it quite a bit in the hospital for kids who don't tolerate a mouthpiece or mask.

Ironically, I just published a post regarding this exact topic.  You can read it by clicking here:  Nebulizer or inhaler: Which one works best for your child?

To be honest with you, even though this is not the preferred method, sometimes you have no choice but to give a treatment this way.  We call it a blowby.  Even though a lot of the medicine is wasted, he will still get 10-20% of the medicine, which is better than nothing.  When there is no other option, this works fine.  Just make sure you let your doctor know because he might want to give a higher dose, although usually doctors are just fine with you giving it that way.

On a side note here, you also might want to discuss with your doctor about getting a steroid inhaler to try with a spacer.  This is actually the best way to get aerosolized medicine into a child's lungs.  In fact, it's the recommended method.  And I hear most children tolerate it just fine. It's also much easier to do and takes much less time.  If you want to read more about giving inhalers with spacer/mask to children check out this link. 

Oh, and one more thing.  If you do continue to give the medicine by this blowby method, make sure you block the oposite end with tape or something so that more of the medicine is not wasted and more blows to your childs mouth and nose.

Any further questions please let us know.