My 6 month old baby won't laugh?

My son is 6 months old, and I cannot get him to "laugh". He smiles a huge smile, and he kind of coughs, but he doesn't outright laugh. I have heard him laugh (sort of) before, but not like I remember my older son laughing. Could something be wrong? I mean, he is doing great with everything else...scooting, smiling, using his hands, almost sitting by himself for long periods...am I worrying for nothing?