my kids went the full range of potty training....the one that never had an "accident" after putting him in "big boy" pants, the one that trained extra early (18 months), the one that was too lazy to tear herself away from her play to use the toilet (she LOVED her pull ups so until I took these away there was NO hope), and the fighter....he like yours completely refused to use the toilet.
What I found helpful for him, was to look at WHY he was "afraid" of using the toilet.....He was afraid he would fall into the big toilet, and didn't like the feel of the potty chair. I resolved that issue by buying him one of those seats that fits into the big toilet seat ( Babies R US carries them as well as ONE STEP AHEAD)....this helped, but we still had issues. I then tried to reward his "good" behavior ( the few times he actually tried) but this did not help....like you I then began the "punishment" round. I then took the stand the if he wet his pants then he must want to still be a baby.....I put CLOTH diapers on him ( tricky I know, but at least I had the control of him being wet, but not quite so easy for him to just change his pants on his own without it being noticed)...he was given a bottle, put back into his crib (we had to pull it out of storage since we had long ago moved him to a big bed) and allowed him only "infant" toys, food, and activities......in a few days he was asking to wear the "big boy" pants, eat and play in the manner that was age appropriate. He still had a few "accidents" for weeks, but they disappeared in a reasonable amount of time. He was unable to stay dry through the night until he was nearly 10.....(this was before the marketing of pullups "Good Nights" for example, sized for older kids) so it meant many mornings of changing his bedding etc... I talked to our pediatrian about the bed wetting and while they were not a great deal of help at that time, things are different now ( this son is grown)....Keep track of how long he can "hold" it, what he is eating or drinking that could trigger his inability to control himself ( sodium in food and drinks, caffeine, artificial food coloring)....cutting them out (or at least limiting) of his diet can help. These days there are lots of suggestions for how an adult can control frequent SUDDEN urges.....check out the suggestions that include behavior modification on these sights...they can help you help your son. I assure you, he will NOT be wetting the bed by the time he is ready to head off to college unless there is something seriously physically wrong......
It is fustratiing for you now, but things will improve!