I think the other posters are right and this is not aggression, just overly enthusistic, untrained greeting behavior. Puppies nibble and lick at grown dogs' mouths in greeting. Some dogs try to mimick this with humans they think are in charge. Usually this gets corrected while the dog is young, but if he was a stray, no one may have cared to take the time or effort to teach him manners. Teach him appropriate greetings routines. Yelp when he nips (sounds funny, but a high pitched "ow!" and 30 seconds of ignoring work really quickly to teach a dog that nipping means losing the attention they want so badly) Good luck!
Dr. Seuss~DLH (brother's cat)~June 2007-
Misty~DSH (my cat & Mooby's mom)-?-
MooBoots(Mooby) ~ DMH(Mom's cat)~July 21, 2008-
Sunshine~ Golden retriever X white german sheperd (Dad's dog)~October 24, 2008-