J.W.
N.,
You're doing just the right thing right now... Encouraging him to use the potty when you do or when you enter the bathroom. He's probably too young to try formal training, but it won't be long from the sound of things.
Both my girls started just like your son, around 18 months. To make it easier for myself, I put them in Pampers Easy-ups (they were the only ones small enough), so we could just pull them up and down when they wanted to try. When we began training in earnest at around 23 months we made the switch to underwear. I used the book "Potty Train in 3 Days" by Lois Kleint. Complete training (when they tell you they have to go) definitely takes longer than 3 days... so be patient. As someone else said, make sure you have the full support of your child's care giver or all your efforts will be futile.
Happy training!
-J.