This used to happen to my little girl everynight. Just as you described. It is pretty scary. To get her to try and take a breath i would gently blow on her face. When she choked she seemed to hold her breath a bit. The good news is it stopped happening so often and it only occasionally happens now. (she is 4 months)
I almost dialed 000 a couple of times, it's pretty scary. Hope it settles down for your little guy too.