Potty Training

I've been trying to potty train my son since April after his 3rd birthday had come & gone at the end of March. Some days were better than others obviously but now he has seemed to have just competely turned his nose up at the idea of going potty on the toilet. He has been in Pull Ups only for about 2 months or so now - we have the regular "big boy" undies for him to wear though he never wants to actually wear them. We got to a stage in the potty training where it was literally a fight trying to get him onto the toilet. He'd be crying, I'd be frustrated. Once we ot to that point, I decided to take a step back and relax about the potty training. Afterall, it doesn't do any good if he (and I) are stressed out about it. I started to ask him if he wanted to go sit on the potty whenever the potty watch song went off rather than tell him it was time to sit on the potty. While stress wise, I believe it was a good idea, progress wise I am beginning to think it was a mistake. It's rare he ever wants to sit on the potty anymore and I am so worried he is going to be 4 and still in Pull Ups. -- Especially when all the 4 year olds I know are exclusively in "big boy/girl" undies and especially when I see friends on FB posting that their 2 year olds are officially potty trained. *sigh* We've tried candy, targets, toys... All work for a few days (if that) and then he isn't interested anymore. I know each child will eventually be potty trained when he/she is ready but right now I don't feel he will ever get there! =( Am I doing something or did I do something wrong? I know boys are harder to train than girls, but I never thought it would be THIS hard or THIS frustrating! I feel like I'm failing in getting him to that "next" stage (meaning the undies stage) of life.