It is a phase and probably an attention seeking thing. Just keep saying no we don't hit it hurts, put him down if he hits you, ignore it as much as possible don't give him a big reaction as this will make it worse, when he is a bit older if he is still doing it you could try time outs. Don't hit him back (although I know it can be tempting) as this will just reinforce this type of behaviour.

Good luck and hope it passes quickly

Rosebud is right when she says it's a phase, but there is one problem with phases, if you allow it to continue it will become the norm.
The main site has plenty of tips, like this one

dealing with tantrums