We were lucky with my son. At 2.5 I decided I was tired of changing diapers. We went cold turkey. No pull-ups. Those things are a marketing design to get you to keep spending your money. Our generation didn't have Pull-Ups and we're all potty trained, right?
I bought some Thomas the Tank Engines trains and some Thomas tattoes. He got a tatto for #1, and he got the new train for a #2 in the potty. Done in two days. I also kept him in the kitchen with no pants on while we trained.
He got big boy underwear that he got to pick out, but I told him if he pooped in them, they went in the trash and would not be replaced. He had one accident, ever. And that was because we just couldn't get to the bathroom in time, so I couldn't blame him.
Make sure your son is getting plenty of fiber and liquids. You want him to have to go so he won't get constipated. That can cause other issues. As long as he's going at least once a day or so, he's fine. If he doesn't want to go at preschool, but goes when he gets home with no problems (straining, hard pellets, etc), he's okay.
Good luck. It will happen.