The frequent wakings are to do with the same thing.
Everyone (adults, children, babies alike) sleep in cycles. In between one sleep cycle and the next will be a period of lighter sleeping. In adults that might involve a brief glance at the clock and changing position in bed. In children and babies it may involve a half asleep shuffle around, a cuddle of a snuggle toy, a grab of the dummy and quick suck, or just a eyes open for a few seconds, look around and flop back down into the next sleep cycle.
This light sleeping period between sleep cycles is biologically left over from caveman days when humans had to ensure they were safe and protected even whilst sleeping. It passes most of the time largely unnoticed.
In your dc, the brief stirring between sleep cycles is often not brief and involves fully waking up, not just a semi-concious stirribg as expected. The reason for this is the child goes to sleep in your arms and wakes up not in on yoir arms and without you even there. Imagine you went to sleep on your bed and turned over to glance at your clock at 3am to find you were lying in the backseat of a car. You'd be fully awake, sitting up and fretting within seconds wondering what on earth has happened.
Once you are out of the night feed stage (which you are) and if you discount things like teething and illness, the reason for frequent night wakes is to do with the method baby goes to sleep. You need to work towards him going to sleep in the position he will stay asleep and also that his comfort mechanism is something he can do in a semi-conscious state on his own - I'd suggest dummy and snuggle toy.
How long it will take depends on how slowly and gradually you withdraw the level of reassurance he needs to get to sleep. This is no quick fix though.
Google "What worked for us mumsnet" for a very informative thread on a similar method.