Firstly, Boxers are known to be bossy. We have one who comes in the clinic that since she was young has been bossing her owners around. She would lay on the owner's girlfriend's neck and growl if she tried to get up, and this was happening when she was like 6 months old, and it's continued on.
Firstly, when it comes to food the dogs should not be fed together. Feed them in different rooms, or very far apart. Secondly, when he went after your other dog I hope you scared him within an inch of his life, because that behavior is NOT acceptable and as the pack leader whether you had to alpha roll him, or smack him [not beat him, just say "Hey!"] it's your job to be fair, but also let him know when he has gone too far.
When it comes to food he should have to sit before each meal, and stay until you tell him he can eat. This is a way of enforcing the fact you are the boss.
I would recommend taking him to an obedience class ASAP, and getting a trainer to help you more. It's only going to get worse if you don't make him do what you want him to do NOW, not when he feels like it or feels in the mood. YOU are the boss, you decide when he eats, what he eats, how much he can eat, when he can sleep, where and when he goes for walks. The truth of it is that he is NOT in charge of his life. He doesn't own any toys, his blanket doesn't belong to him, it belongs to you because you are the alpha, apparently you haven't pointed that out to him.
Does he let you trim his nails? If not that's another thing to work on because that's another thing he should be letting you do.