Erg. Panting can be overheated, in pain, or having difficulties breathing. If you have or can readily get (as in, soon) Rescue Remedy (most health food stores and even good pharmacies these days), a drop or two in his water wouldn't hurt and may help. They have a pet version now that doesn't have the alcohol base and so doesn't have to be put in water - can put a drop on a piece of watermelon or anything else he'll accept. It's used for helping to relieve stress.
JJ
JJ