I do have, however, some information and advice that may frame the sleeping problem differently and some tips on improving nighttime awakenings. First, a brief developmental framework. Most infants "settle" themselves or begin sleeping through the night between 3 and 6 months. Unfortunately, as you are presently experiencing, an increase in nighttime awakenings is very common during the second 6 months. These problems may still exist or not even develop until after the infant turns one. In fact, up to 33% of 1- and 2-year-olds are still waking up at night to an extent that is concerning their parents.
I am telling you, despite your first child's experience, that your son who is driving you crazy is developmentally right on schedule. I bet I didn't make you feel much better with that knowledge but I had to frame your problem in an appropriate manner.
I would suggest, especially since he has recently demonstrated that he can sleep through the night on occasion, that upon his nighttime awakening you let him fuss for a few minutes(3-5); if he is not settling himself down in that time period go into the room and soothe him by patting his back, rocking him, whatever he has associated with getting to sleep in the first place. Spend only a few minutes doing this and then leave the room, regardless of whether he has fallen back to sleep or not. Continue to respond like this on every subsequent awakening but shave a little time off your stay each time. The idea is not to see how long your son can cry but to develop new associations for him going to sleep on his own and settling himself after awakening on his own. You need to parallel your leaving his room whether he's awake or not after an awakening with initially putting him down to sleep when he's awake. What we're after here is developing, without fear, an ability for a baby to get to sleep on his own when placed in bed awake and an ability to self-calm himself back to sleep after an awakening. Good luck. Hang in, better sleep is coming soon.