Hmm. I'd take him on a 10 mile hike and then see if he was still interested in the mirror. :)
The dog thinks that there is another dog, (he makes play posture at the mirror).
He doesn't recognize himself in the mirror.
It is always a problem with most animals, especially fish and especially males.
Only great apes, dophins and elephants have been known to recognize themselves in the mirror, humans too.