At about the same age DD was having the same number of feeds as your son. She self weaned at 19 months.
I found with DD the easiest way to help her start dropping feeds was to make sure that she ate her solids first and then offer a bf a little while afterwards. Before she could ask for the feed I started to check if she wanted other things first - she knew the word 'no' so I'd go through the list of "do you want ..., ...." etc and gradually work down to mummy milk - if she didn't want what I offered she'd say no if she wanted it she'd nod her head. As she started to talk more she started asking for 'mummy milk' so I'd only feed when she asked - I think there were times where I was offering a feed without her really wanting the feed so by feeding when she asked it helped a lot in the weaning process.
With DD her feeds tended to match a certain timeframe so we gradually got it down to she'd only want a feed in the morning and then it was just a case of delaying that feed a little at a time (she'd ask and I'd say "soon, mummy just needs to do x") eventually it got to the point where several days had gone by and she hadn't asked for a feed at all and I considered that to be when she self-weaned. Yes, I'd guided her a bit in getting started on the weaning process but if she wanted/needed the feed I was alway happy to provide it, she just got to the point where she didn't want it any longer.
Hope that makes sense.