I have never tried to train a boy, but I have an 11 year old and twin 4 year old daughters.
The 11 year old, potty trained right around age 2, barely any issues. The twins were another story. I started trying before they were two. I finally quit pushing when they were around 2 1/2 or so, and then right around the time they turned three I started at it again.
The thing that worked for me, was that I took time off work and stayed home with the girls for 5 days straight. We didn't leave the house. In that time, I put them in panties and did not use diapers or pull-ups. I made them sit on the potty every hour. They wet and messed themselves over and over the first day or two. There was a lot of laundry and a lot of carpet cleaner being used, but by the third day they were doing GREAT. By the 5th day they pretty much were trained. They did still sleep in pull-ups for another 6 months, but I just made sure they didn't drink anything significant after about 6:00 p.m. (a little bit of water before bed) and they really didn't have a lot of trouble with bed wetting either.
I also heard of putting a bowl of cheerios on the back of the toilet and having the boys throw a few in the toilet and having the boys play "sink the cheerios." A friend said she couldn't get her son OUT of the bathroom after that.
Involving Dad would be good too. I would imagine boys would relate more if Dad was showing him how and encouraging him. Also, if your older boys would get involved, I'm sure that they would be a huge help too. My older daughter was great at getting the twins excited about going on the potty.
Stay strong. I think the previous poster was right, you should probably take a break at this point and try again later. Maybe you could build up to the day, somehow, and say "5 more days and you're going to stop wearing diapers. 4 more days...", etc. Try to get him excited.
I feel lke it's harder to concentrate on training your kids once you have more than one! There's so much more going on! Try to stay patient. If he's healthy and doesn't have any medical issues, then I'm sure he will get there in his own sweet time!
Good luck!