P.M.
Expectation that your son should "stay trained" after he's had some success can actually set the process back and foster a sense of failure or resistance. Families who allow the child to lead will still end up with fully trained children just as soon as the child is mature enough. For many kids, that will happen somewhere between 2.5 and 4.5, with boys tending toward the later age.
Potty readiness is not only dependent on body and nervous system maturity, but also emotional readiness. Children with initial home runs are sometimes, but not always, emotionally committed to the ongoing responsibility. The obligation just looks too large or onerous for many kids, so backsliding after a period of apparent success is common.
Can those kids be cajoled or maneuvered or punished or bribed into ongoing performance? Sometimes, but that's really not the same thing as the child successfully achieving that developmental milestone, which can become totally voluntary only when the child is both physically and emotionally prepared for it.
When parents aren't having good results, it's a good idea to be sure the child is ready before proceeding. Here are some useful checklists: http://www.parentingscience.com/toilet-training-readiness...)
Whether or not your son is there yet, you can still continue with whatever "pre-training" he's responded to – helping him become acquainted with the potty, perhaps practice sitting, reading stories and watching videos about using the potty, role-playing, potty parties with his favorite toys "using" the potty, etc. All these things can encourage the child to think positively about the process, and may promote emotional readiness.
Since your son really wants the undies, it could be that he's on the verge of discovering readiness. If he's close, what's an extra load or three of little-boy underpants, and some wiping up around the house? Changing diapers is time-consuming and expensive, too.
So you have at least two options, both of which are reasonable: continue to let him wear underpants, and each time he goofs, cheerfully remind him that pee/poop goes in the potty, and you are confident that he'll figure out the signals real soon. Or put him back in diapers and coach him for awhile longer on recognizing the signals that tell him he needs to get to the potty. I've seen both approaches used with success (which can happen in a couple of weeks or a few months, depending on the child).
I hope you won't coerce or shame your son with the message that he's ruining his underwear. That will not inspire confidence or a positive attitude, and it's simply a lie. Besides, once they're "all gone," how are you going to explain that they "come back" when he's able to use the potty?
Do all you can to keep your attitude upbeat and positive. Avoid comparing your son to older child – all kids are different, physically and emotionally. And if your son detects your annoyance, that will also undermine the sense of joy and accomplishment he can and will have when he finally succeeds.
I grew up in a generation that rigorously pushed potty training on kids almost as soon as they could walk. My mom remembers a sort of competition among mothers who wanted to brag that they trained their kids young, as if it were a mark of parental excellence. This approach also produced quite a few people with serious emotional problems to work through as a result of training that was neither realistic or child-centered.
