Use your thumbnail to get him off you, and he will stop pushing into you.
As for nipping, keep stiff brush in hand and let him run into it. Or horseshoe nail just barely sticking out between knuckles, (like you do car keys) and let him run into that.
Do NOT use force, or look at him, just move hand so that he bumps into brush/nail tip. Ignore him otherwise and keep brushing or whatever.
And the fact that he is constantly battling you over "authority" tells me that you have not been serious enough about ending this type of thing.
You need to reassess your handling of this horse, as well as anyone else who is fooling with him. If any of you let him lip you, smell you, or in general behave like this, or any of you baby him, or feed treats. You are causing this behavior basically.
And use your elbow points too, if leading and he comes into you, poke him with point of elbow in his neck/shoulder, if he swings head into you while leading, use fingernail to remind him to get over.
It does not take much to fix this, but you have to change your handling, and get serious about it first.
Horses make me a better person.