DS is five months old and still hasn't laughed. Should I be worried?

He squeals and screeches and makes all sorts of noises - but he's not sounding out consanants either. He used to want to "chat" with us, but he's not really interested in that much either anymore. And it's more moans and noises than trying to mimic real language.

He smiles and makes eye contact, responds to our voices and other sounds/noises, sometimes makes these weird screeches/guffaws on an intake of breath that sound like goofy attempts at laughing, will smile and throw his head around in silent pleasure - but no belly laughs, no giggles. I once managed a chuckle out of him, but I've never heard it again, no matter how hard I try.

What age did your bubs start laughing? And should he be attempting to mimic us and make real sounds by now (like baba, mama, dada, etc)? Not as in, his first words, but trying to put together familiar sounds?