Teething could absolutely upset his tummy and cause diarhrea.  We all forget that they are babies too!  He could be feeling mopey and tired as well.  It takes 1-2 years for a dog to gain full physical maturity, yet it takes humans 15-18 years.  Their bodies grow at such a rapid pace and we don't take that into consideration.  That's one of the reasons they say not to have them running on hard pavement until later, because it can do so much damage to their legs.