Baby laughs?

My baby is 17 weeks and really doesn't laugh. He smiles a lot and sometimes squeals. One night last week I got a real gruff laugh for just a few seconds and nothing really since. At work there are people who show me videos of their grandkids laughing and laughing and 2.5 months, three months. Now here we are at four months and still nothing. Also he rolled (both ways) a few times really early (between three weeks and 2.5 months) and now he doesn't anymore. He scoots onto his side but nothing. I know we shouldn't compare our babies but I'm just curious if there are other four monthers not laughing or rolling yet.