I know that you dont like me much but I thought I would put in my 2 bobs worth anyways. Please dont take any of this offensively!
Firstly, I totally agree with one of the previous posters in that this is normal behaviour, toddlers dont share, they are programmed like that! I know we all take it for granted but has your neice ever been shown how to share?? Maybe she doesnt know how?? They dont come born with this skill, it actually needs to be taught.
I know it would be horrible hearing someone talk to your son differently (worse) than what you do but in reality, it happens and theres nothing any parent can do to stop that. When you say that you want him to be able to stick up for himself, you mean whenever a child says something "mean" then he should belt them up?? I have no problems with you wanting to get him into boxing but just make sure you send the right message to him. Violence doesnt solve anything! I totally agree that he has every right to defend himself if someone is physically touching him and he doesnt like it but if someone is saying something that he doesnt like, it doesnt give him the right to fight them, just ignore and walk away. Children can be downright mean at times, your son wont be nice to everyone either but as we get older we just start to tolerate everyone even if we dont like them.
Anyways, thats my 2 bobs worth, I didnt mean to offend you if I did!