Aggressive Behavior

by Connie L
(Puyallup, WA)

Leo is a 15 month old, neutered male Aussie. He exhibits aggressive behavior with strangers. Sometimes as soon as he sees them he is aggressive and wants to charge and bite. Other times he seems fine, then when they come up to pet him, he will let them for a few minutes and then become aggressive. Sometimes he will charge and not break the skin but leave a bruise.
He is fine at the dog park, but we are wary of taking him places. Any suggestions that don't involve an expensive dog trainer that we can't afford at this time?