I used to teach the "potty training class" at a preschool. I have many ideas and some that I used at home myself.
-Washable toys that can be used on a small table (puzzles, books, cars, dolls, etc.)
-Bin to store them in
-TV table to keep in bathroom
-Digital timer (one for each of them is great)
-Thick training underwear (save the character undies as rewards)
-Doll that "goes" or has a hole in the crotch (I used a bathtime Cabbage Patch Kid cause it looked like it peed when you lift it out of the water)
-Reward charts, stickers, small candies (M&M's, Skittles, Sweet-tarts, Smarties, something you approve of)
-Tootsie Rolls (this one is important)
-Be patient; it sounds like you are 1/2 way there (never get upset at accidents just use it as a learning experience and have the child do as much of the "work" cleaning it up as can).
-Be consistant; if you start the process and then stop it, you will teach them that it is okay to go in their pants sometimes. (Your goal is 100% success.)
-Use the timer to gage the time of sitting and time between visits to the potty.
-Use the TV tray to place toys in front of the child to keep them busy and help them to relax.
-I agree that the training pants are too close to diapers and are really a "waste of money".
-Reward with one candy for what the child does better and 2 for what they need to work on.
-Teach your son the art of peeing on a tree (this is Dad's job).
-"Show" them how to go (take them into the bathroom with you and talk to them about what goes on). Also, use the doll to explain that after you drink, you pee. This is a great visual aid for the parent who does not want to "show" the child.
-Use the tootsie rolls to make it look as if the doll has "pooped". Have them ready and unwrapped ahead of time and distract the kids to put them under the doll. I used to say, "let's give our friend some time to sit and go poo-poo." (This also shows that sometimes you have to sit for a while.) All leave the bathroom. You go back when they are not looking and put the tootsie rolls under the doll, then call the kids back to see that the doll was successful. Be sure to get VERY excited and reward the doll in front of the kids.
I hope it helps.