It feels like a huge regression to you, but your son may simply see it as needing to regroup for another charge. I know lots of moms who have put their kids back into diapers (in despair, of course) after the newness and excitement of potty training wore off. But when the kids themselves realize that they were going back to a system that required them to submit to diaper changes when they'd rather not, they start to recognize the benefits of a quick sit on the potty.
When a child has made that mental connection, assuming he has developed the needed degree of physical control, he's very likely to decide he prefers the big-boy pants. At that point, it's the child's decision, not the parents', and it tends to stick.
For boys, this decisive move often comes later than for girls. You don't say how far into 3 your son is, but it's not at all unusual for a boy to reach that point somewhere between 3 and 4. Sometimes it's only a matter of days or weeks after being allowed to use diapers again.
Since you're still using cloth pants on him in spite of the frequent accidents, I'm guessing you're doing a great deal of cleaning up. He's probably also feeling a great deal of tension and disappointment from you, which means he's more than likely feeling that as his primary (non)motivator. I'm guessing he needs to have a chance to be totally in control while he works this out.
If I were in your position, I would tell my child something like, "You know, some kids get very nervous about using the potty when they're afraid they might fail. Just like some babies are afraid to walk for awhile if they fall down. But I trust you to figure this out just as soon as you're ready. I know you'll want to use the potty when you're ready, just like you wanted to walk when you were ready. I'll ask you every morning if you want diapers or big boy pants, and you will get to decide."
I've seen this approach work well, and sometimes very quickly.
You might find other helpful information at this extremely informative site:
http://www.parentingscience.com/toilet-training-readiness...