Well, I never had this issue with Charlie, but I took his manhood away when he was 8 months (on the day). He never started this either, some dogs are more prone to is than others, Neutering him can help a lot, but you also need to be doing more than just telling him no. He is obviously ignoring you. Try with pushing him off forcefully (nothing to hurt him but to startle him). You need to establish that you are the Alpha not him. If you don't start doing something now you will never be able to control him, he will control you. I know a lot of people are againt a quick swat and a very firm NO, but it will take his attention off what he is doing. Boxers are very sensitive so this should have a huge effect, and you dont need to swat him in the head (this can cause hand shyness), just on his side works well.
As for neutering, my father and brother REFUSE to neuter their male dogs because they feel bad.

, of course it helps a lot with certain behaviours, agressiveness (sometimes), hyper activity (to an extent) and can actually extend their lives. So just something to consider.
