It's true that boys change at about that age. I don't know whether it's testosterone, or peer-influences.
Ds1 was such a good, easy-going, obedient boy that we worried there was something wrong with him. He was better than textbook. It was quite a shock, when he began Y1, to learn that he had turned into a leader in mischief in his class - he once stole his teacher's shoes right off her feet!. He also started answering back, being grumpy, losing his temper...all the typical behaviours of childhood.
But even though he seemed to have undergone a personality transplant, when he wasn't being a monster he was the same sweet-natured, easygoing, rational child he had always been. (Sometimes for only a few minutes a day.) And, even when he was being at his most challenging - whether at home or at school - the teachers still considered him a pleasure to teach.
So I would still say don't worry to much about it. Talk to the teachers, see how he is at school - do they think there's been a change, and is it a problem?
Respect his feelings, and his need to express negative feelings, but don't fuss over them.