I have been there... My son only is just now understanding the potty business and we only really became aggressive with it in May. He will be four in August, beginning preschool in September.
I have learned for myself that you have to have patience throughout the whole process. Our biggest reward is a Zoo trip as we have the family pass and it is not an extra cost to us. I will reward him further with a small toy at the gift shop. I found this worked the best for us (as we too did the stickers, books, etc.).
Many kids develop at their own pace. As is sounds like you have all boys, you are aware that they do learn this business the slowest compared to girls in most cases - but still at all different stages.
If your son has not eliminated within 10 - 15 minutes, then I would take him off the potty. Otherwise, he may be viewing it as 'punishment' and not a learning/ training experience (or very pleasant).
Something to keep in mind: many children FEAR that more than poop will come out of their body when they are on the potty. Sounds silly, I know... but they are new to this! :)
Patience, Love, Understanding, Talking... all of this will help your son. Plus, he may be bound (constipated) so it could be hurting him? This occurred with my SIL's son. I have the advantage as I know how rid this for my son and within 15 minutes he will expel on the potty and without too much discomfort.
However, he sounds as if he is 'on course' to me in learning.