There is an "Inhale" morph for the chest, might be in the morphs ++ set, If you don't have that, you  could use a magnet on his chest to create chest expansion when he inhales. In the magnet properties tab select "add element to deform" then add in collars and abdomen. Do the same for any clothing items.
If you need more visible breathing, then add just a little backwards bend to the chest, inhaling tends to straighten the  back slightly