Well, if you were watching him and the water was shallow enough, I would say he probably was not about to drown. One of my dragons, does this almost every time she is in the
bath. She puffs up to float and will fall asleep, if I do not prop her head up she would probably drown herself cause she will not move. So I think your guy will be fine, but watch this, as if he is anything like mine this will happen a lot. She will actually stay puffed up until I remove her from the
bath, just like yours, breath heavy for a few seconds, and then be fine.
I have used a plastic bottle as a float or a towel or my hand in some cases to keep her head above water. No sure why she does this, but it happens, and is normal.