Honestly, you know your son better than anyone, but maybe its a control thing. Like the "Throw it down, Mommy picks it up Game".
If he is pointing to himself when he needs to go, then he understands the urge to go. As with any child learning, just go with his pace, but be careful of sibling jealousy and reinforce that you are the boss and always go with him to the bathroom. Think of it this way, "would you let him go to the bathroom in a public place on his own, or would you insist on taking him to the bathroom with you?"
Some kids just need a little extra push to potty train.
I would suggest giving him a penny every time he goes in the potty and when he has enough collected let him buy something with it. You could also put cherios in the potty and let him try to sink them. (I just let my son take a few coins from a cup of change and put them in a big jug. By the time we were done, he had four jugs full.) Of couse he was only allowed one hand full for each time he pottied in the potty.
It gives little tots great incentive to get a rewarded when they do something good. And of course.....Lots and Lots of PRAISE is also a plus.
Make a big deal out of it when he does what you want him to. Clap, say "Yea....", just whatever gets him excited. Maybe buy some big boy underwear and tell him they are just for "Big Boys". When he doesnt do what he knows he is suppose to put him back in a diaper, tell him he isn't being a big boy, or tell him that makes Mommy sad.
All of my 4 kids were potty trained by the age of 2. "And it was done at there own pace (like your son) and not because I pushed them. I just took there lead and followed it. Some days were good, some weren't. I never made a negative thing out of it.