P.M.
It is sometimes possible to train boys this young. But it's usually a case of the parents training themselves to remember frequent potty breaks, learning to recognize their child's "about-to-pee" behavior and "most-likely-to-poop" schedule, and being willing to put up with frequent accidents for months. Many parents don't consider the child trained when it's the mom who's actually trained herself, even if the little guy cooperates cheerfully.
And even when he figures it out, it's not always for keeps. Once the child realized the demand for consistent success stretches on for the rest of his life, he not infrequently regresses. Regression is also extremely common when there's a major change in the child's life, a move, loss of a close relative, a new daycare situation, illness, or a new sibling distracting the parents, and many experts recommend NOT pushing on until the dynamics are stable again.
Every family I've known has had the best results with waiting until the child wants to train. And kids do, just like they want to learn to walk and talk as soon as they're ready. It's excellent to talk about potty training and what the body does, and to experiment with sitting on the potty, it's great to play potty games with toys, puppets, and occasionally the child, it's great to read books and watch videos, to let him watch you use the toilet, to observe how much easier time he'll have when he learns to use the potty and doesn't have to stop for diaper changes. (I thought of this as "pre-training" with my daughter.)
And at some point, he'll start showing more curiosity and indicating that he wants to try, perhaps even voluntarily sitting on the potty himself. He'll probably be thrilled by the whole idea of big-boy underpants. That's when you can let him know you'll help him remember; let him help you work out a plan so he's got some control, and go for it. Kids will often train in a matter of days at that point, with fewer accidents every day.
Children who are ready for this step toward maturity take great pride in the accomplishment. Charts and reward systems aren't even needed. Children who are pushed into it tend to become irritable and resistant, or even worse, begin to feel a sense of failure and frustration.
Here are some helpful readiness checklists: http://www.parentingscience.com/toilet-training-readiness... If you decide to go for it even if your son hasn't yet shown initiative or that he can reliably recognize his urges, be prepared with a timer to schedule regular visits, toys and books for the potty, and a supply of cleanup rags.
Also, be aware that poop training and night training are sometimes separate steps for some children. Children can not help this.