"Regression" is symptom of stress in a child and their inability to cope with something. And also because at this age, they do not have the full capacity to cope nor understand and articulate their feelings and thoughts, accurately.
For accidents, I would NOT punish him. He cannot help it.
It will only backfire.
For night time, keep in mind that all night dryness... is biological as well and has to also do with the myelination of the nerves. That he cannot control. And FULL night-time dryness... is not attained until even 7 years old. Biologically. This is per our Pediatrician as well. Normal.
"Accidents" will happen no matter what age. They will not be perfect. My Hubby, even when he was older than 7, would have accidents. My daughter is 7, and she on occasion has accidents as well. Normal.
I just use a waterproof bed pad directly under her, which I got from www.amazon.com I have about 4 of them, and if it gets soiled, I just switch it out, put a new one on the bed, and that's it. My daughter even had a dream once that she WAS on the toilet, until she felt herself wet. Its okay. They are just kids. If my daughter has an accident, its NO biggie. She just changes jammies, I put a new pad under her, she goes back to sleep. It does not make sense to me to scold the child about it... they don't do it on purpose and it happens in their sleep which they are not even doing on purpose. Nor do I wake my kids at night just to pee. I rather them sleep.
My son is 3.5 years old... and only recently has he been ready for pottying. I waited until he was ready... and then he pretty much trained himself. But, he still is not 100% about holding it for long periods so a little pee seeps out.. and he is still wet at naps and bedtime. So he wears a diaper at night and for naps... and when we go out, and for long periods. I go according to his cues and bodily ability. I never scold/punish for my kids toileting.
When I was pregnant with my son, and once he was born.. my daughter who was then 4, regressed and started having accidents. NO biggie. We just talked to her, comforted her, did not shame her, we told her we understand...and one day on her own, it was resolved. Naturally. It was just that HER life had "changed" with a baby in the house... and she was adjusting.
He is only 3... and this is normal. Don't pressure or punish for it. They can't tell their bladders what to do... and pressuring will just make a child get hang ups.
Here are some links about it, from Parents Magazine:
http://www.parents.com/toddlers-preschoolers/sleep/bed-we...
http://www.parents.com/kids/sleep/bed-wetting/child-wetti...
http://www.parents.com/kids/sleep/bed-wetting/bed-wetting...
http://www.parents.com/parents/quiz.jsp?sssdmh=dm17.43667...
Good luck.
Susan