I would disregard the advice to try bullying your dog into believing that you are the "leader". That's not how canine leadership works, and your dog won't believe you're his leader that way, he'll just believe that you can be unpredictable and nasty. If you want to be the leader, then lead. Bullying gets you nowhere, and aggression begets aggression.
You might like to take a look at this previous thread, that contains some very good links to articles on leadership and dog training:
http://boxerworld.com/forums/showthread.php?t=71282
As for your actual problem here - have a think about how you are actually reacting when he does this. He is an adolescent boxer, and they are notoriously pushy and demanding at this age. It sounds very much as though your pushy adolescent is trying to
play, and he is demanding your attention. So how are you reacting to this? Are you giving him attention? If you are (and negative attention is still attention) then you might like to try a different approach.
For a start giving him an alternative thing to do is a good idea - redirect him to a toy of his own. If that doesn't work (and initially I don't imagine that it will) then you need to ignore him. Don't give him the attention that he is seeking, or anything he can interpret as engaging in his game. Giving him a time-out is one possibility, but you need to give him attention in order to do that. So a better idea is probably simply to get up and leave the room, shutting the door so that he can't follow. Don't speak to him, acknowledge him or even make eye contact. Just get up and leave for a few minutes. That may not seem likely to be effective to you - but in fact it is. It is isolation for the dog and the precise opposite of what he was trying to achieve. Instead of attention and his game, he gets isolation and NO game. And if that is the consistent response he receives, then he will give the behaviour up because it doesn't work.
Note that initially he might try even harder, since behaving like that has got him what he wanted in the past - but if you're consistent and just get up and leave, then he will become convinced that that sort of behaviour does not get him what he wants, and because it doesn't work, he'll give it up.
Another thing you can do when he's being a bit pushy is to engage him in a short training session instead. That will give him something to do and to think about, without actually engaging in the game he wanted and simultaneously reinforce your role as leader. Taking him to obedience classes is also a VERY good idea. A teenage boxer is an obnoxious one, and ongoing training is very helpful in keeping him in his place in your 'pack'. At this age, ongoing physical and mental exercise is very important - he needs to have things to do. And a tired boxer is a good boxer

(note that is mentally, not just physically).
Have you ever read "The Dog Listener" by Jan Fennell? If not, it is a resource that you might find very useful. It deals specifically with establishing and maintaining a leadership relationship with your dog. Could be very useful for you at this time.