I started potty training my son when he was 18 months old, because the daycare he was going to required it by age 2. Now at 3, he still only will go if the daycare provider tells him to. When will he start going on his own?
Children need reminders to use the toilet for quite a while. Children get so involved in playing that they have trouble thinking about their bowels and bladder while involved in play.
Rather than telling him to go to the potty, say to him (really!), "Think about your bowels and bladder. Are they full? If so, you need to go to the potty." With this approach, you're putting the burden on the child to listen to and read his body's signals for needing to use the toilet.