I have 2 male boxers too. One is 18 months and neutered, the other is 13 months and intact (soon to be neutered). They get along really well. They play together, eat out of the same bowl, sleep together, play tug-of-war, chase each others tails... well one stump and one tail. The older one started showing the younger one the house "rules" as soon as he got home. They are amazing together... no growling, no fighting, both have very gentle personalities, maybe that helps?
Good luck with the new pup!