Aggressive behavior in my Boxer

Question
We started off with  with a Beagle(3yrs old), a Puggle (2yrs old) and we decided to add a little bit more crazyness to our household by getting a Boxer pup...Everyone got along great at first...once the Boxer was out of his puppy size and got bigger than both of the other dogs, he started attacking the beagle...to the point of stitches and hospitalization..((All dogs are neutered and male))No problems at all with the puggle.  He has no aggression to the family..kids ages 7 and 1..and other than his issue with the beagle, he's the best behaved dog we have had by far. We decided things were a little too crazy so we found a new home for the puggle and after the most recent and worst fight between the beagle and boxer we have decided to let the beagle live with my mother in law..I dont want him getting hurt anymore, and he's not so great with the baby. Im not sure what to do now. I fear that if we socialize him with other dogs, which I think he needs, Im scared he will attack. He has shown slight aggression to other dogs in the past, however not as bad as with the beagle. He also is a bit afraid of strangers, particularly males. Its not like we kept him in a hole while being a pup, he came everywhere with us, but once he started behaving strange we got nervous and stopped. We dont want him biting anyone.  Any suggestions on what to do now?? Im lost. We love him so much, he is so loyal and loving and great with the kids that I would like him to be around for a very long time with us...but dont want anyone getting hurt by him...

Answer
He is exhibiting the alpha domination which he will defend.

You will need to socialize his with people (especially males) and other dogs as well.

To do so, buy a bike horn or loud whistle, (I prefer the loudest, most obnoxious sounds) and leash him.

Invite a male friend over to volunteer this training session.

Leash him, invite the male friend inside, and the exact moment the Boxer gets aggressive, blow the whistle or honk the horn and verbally reprimand him in a stern voice using simple, easy to understand commands, and pull on his leash putting him next to you in your control.

Repeat this training exercise until he stops the aggression.

Then, add a male friend bringing over his dog and repeat.