Is the dog exactly 5 m/o ? if a little younger or even not it very well coulda been his puppy teeth and not his perm ones....take a good pic of his mouth/teethe and post it....look real close and see if his grown teeth are comming in through the gums.
X2
Thats about the age their new teeth start coming out, maybe his puppy teeth were the ones that got ripped out.