My oldest was 3 1/2 when she finally trained successfully, but only because her infant sister came home and we reinforced the concept that diapers were for BABIES and she was a big girl and now needed to wear big girl underwear.
My second daughter was 2 when I watched an episode of Dr. Phil that gave tips about potty training in a day. I certainly didn't want to wait it out until SHE was ready like he sister, so I tried it out, and it worked!! Basically, he advised to take the potty out of the bathroom and put it in the family room or other room they spend a lot of time in for easy access. He said to explain that you were taking off their diaper, and that meant that they could go potty whenever they felt they needed to. My daughter chose to sit on the potty to watch her favorite a.m. show -- Sesame Street. While sitting there, she tinkled a small amount in the potty without realizing it until she heard the sound it made! She jumped up to tell me about it and I praised her up and down. (Dr. Phil's advice too.) He also said to make a big deal out of it with other family members on the first day it happened by throwing a "potty party." We did just that. My husband brought home balloons and a couple of very small "gifts" from the dollar store. I told her that we would have a special pizza dinner (her favorite) and sundaes for dessert. Her older sister told her over and over what a big girl she was to go on the potty, and my little one just beamed with all of the attention!
The next day she went to the store with me to purchase her "big girl" Sesame Street underwear, and she couldn't wait to put them on. We kept the potty in the family room during the day and continued to reinforce her by encouraging her try to go after meals and before bed. We put a pull up on at night, but for whatever reason, she didn't wet them.
Another idea that helped was to let her get some special smelling handsoap (especially if she can pick it out) that was just for her to use to wash her hands after using the potty. Every time we saw someone we knew, we let her tell them about her accomplishment. If your son doesn't have his own stool to reach the sink, getting one and painting his name on it will inspire him too. (It's also handy for making him tall enough to "hit the bowl" when he goes too...)
Best of luck -- hope this helps!