Nope, early masturbation is how boys learn to control their ability to orgasm, helps them operate sexually as adults. If he's flogging himself a bit violently you may have a talk with him about it ask him if he's ever hurt himself. Maybe be a good big brother and buy him some lube.

its normal
he is pubert now and the force of hormonal growth in him is forcing him to do that.
it happens with every one
it happened with me and even with doctors, god has made us like this. its our own mother nature, so dont worry
its very natural

im 11 it is ok

At 11 you might want to ask yourself why you want to masturbate, but as long as you're being careful to use lubrication and you're not letting masturbation get in the way of other things in your life there's no real harm in it.