I cringe every time I hear that people say that. To me it's an excuse people use when they can't control their kid, and most of the time they can't control him because they haven't tried.
Yes, boys are different. Yes, boys have more energy. Yes, boys in general are slower developing verbal skills than girls, so are more likely to express themselves physically. But if your kid hits mine to try to get a toy or get by or whatever, don't just give me an embarrassed smile and that crap line about boys will be boys. Discipline him.
I'm no expert, but I do have some experience after 7 years of this parenting gig. My now 7 year old had a period when she was about 18 mths to 2 years old, when she would bite. She only bit me. Our pediatrician assured me that was because she trusted me. She knew I'd still love her even if she bit me. Gee thanks kid, I'd prefer a hug :) Timeouts took care of that one pretty quick.
Her younger sister had a phase around the same age when she would hit her older sister to try and get a toy from her. Again timeouts, problem solved.
My 2 year old son will occasionally hit his sisters and gets timeouts for it, and his behavior is improving.
My point is if you nip the bad behavior in the bud early enough, they will know better. Most kids use physical aggression because they haven't learned how to express their feelings with words. Start them young and it'll be much easier to either avoid bad behavior later, or correct it. My kids have never hit anyone other than a family member, which still isn't great, but I have a feeling that if we hadn't disciplined them when they did something to one of us, they probably would think biting or hitting are ok, and then do it to other kids.
So don't give me that crap about boys will be boys. If you let the bad behavior continue, your brat will continue to be a brat, and the only one you'll have to blame is yourself.