[Dog_Obsessed]

I would recommend getting it checked out by your vet. It could just be that he is burning more calories than he is getting, and if he seems otherwise healthy than there is probably not an issue, but weight lose can be the first sign of illness, so I would just make sure it is nothing. Also, is he consistently losing weight or did he just lose a few pounds an then stop? If it is the second one then it is probably not an issue.