Wait until he turns 3. Take away the diapers completely. Put him in undies. Stay home for 3 days. Take him to the bathroom every 1/2 hour in the beginning. The second day take him every hour. He should "get it" the second day. He will have accidents. Stay positive. Have A LOT of underwear on hand. By day 3 he will be trained. Keep the diapers at night until he wakes up dry for a week or so. Good luck! I trained all my kids in 3 days this way.
I think you will know when he is ready, my 18 month old already says poop and pee and wants to sit on his potty. I let him and now take his diaper off. I still think its early for him, but if he is always wanting to sit on it, it won't be long!