get rid of the diapers. Put him in underwear. He'll fight it, and make lots of messes, but he will get the hang of it soon enough.
My son loves his big boy underwear! On days when he resists potty training, he has to wear the plain ones with no fun characters, and when he is doing good, he gets to wear his fun underwear with Spiderman or something on them.
That is how I finally got mine to wear underwear and try really hard not to go in them. Also, constant reminders to go.
Also, when he went in his diaper/underwear, he had to take the diaper off, dump the poop in the toilet and flush it and clean up. That got old really fast for him.