Aggressive 2 yr old aussi

Question
My 2 yr old aussi shep. has become very aggressive towards other
male dogs.  This is what I've noticed: Usually the dog is a bit taller (he's never been aggressive towards smaller dogs)he starts to growl then it's a full out fight. I can't say that he's drawn blood but this morning he managed to knock over a young (10 mos.) huskie who was trying to play with him, twice.
I believe he was the alpha male of his litter and as a young pup he used to growl constantly but would never bite. I corrected his behavior or so I thought by just saying no and he eventually stopped. He's very people friendly and has never had any issues there. I did notice that he's become aggressive towards some of the female dogs that we walk with. (He has been neutered).
He loves to run and I walk him twice a day for about 1 hr or longer on each walk. We have many trails that he can walk or run on. I'm very upset in thinking that I may have to muzzle him but realize that if I do that he won't be able to protect himself and I realize that won't stop his aggression.
Any advice would be greatly appreciated............

Answer
There's no such thing as an alpha male - I know, you've heard the term a lot, but the man who coined it has recanted and stated he was mistaken.  His name is David L. Mech and you can find his recorded statement on YouTube.  In any case, I can't say for sure what is going on with your dog, but a few things come to mind.  Sometimes, when dogs reach the age of social maturity, they no longer appreciate physical play as they once did, or they prefer to play only with very familiar dogs.  Some dogs have fears about certain classifications of dogs, so perhaps your dog is actually fearful of the larger ones and assumes that the best defense is a good offense.  Another possibility is that if your neutered male is encountering unneutered males, he may take issue with them as rivals (after all, they smell like boys!) - I would say based on the fact that he's had issues with females, too, it's the least likely scenario, but still possible.  Some dogs develop health conditions that make them testy, such as hip dysplasia, or hypothyroidism, both of which are prevalent to some extent in the Aussie breed.  If your dog was previously non-aggressive and this has developed lately, the first step is to have your veterinarian do a good clinical exam, plus a full thyroid panel (the panel is about $100 in my area, but if the dog needs medication, it's inexpensive).  If the vet rules out a physical cause, then you need to look at behavior.  You can find a trainer who will use positive methods to effect behavior change at the Pet Professional Guild web site.