My daughter is like this.
She is 3 and a bit now, but first trained just before 2 years old {at her own instigation, to copy her older brother}. I put her back in nappies the first time she regressed, but persevere with undies now since she is constantly back and and forth with it.
Do you think your new baby was the starting point, is your son 'regressing' in other ways?
I can't put my finger on the reason with my daughter, likely a lot of things {we don't have a new baby, she is the youngest} but she really wants to be the baby. For example she refers to herself as baby, likes to make crying noises and baby talk {IE well below her current vocabulary} etc.
In her case I think the pant wetting is part of this, so I've been trying to reassure her on that count.
The other part of it, for her, is that she really wants my constant attention. The pants wetting is very pronounced when I am doing something else that requires my full attention, for example trying to help my son with reading, or during our 'quiet time' in the middle of the day where I do things like make phone calls, or surf on the internet, while they watch TV.
During those sort of times, she will constantly {say every 10 minutes} ask me to take her to the loo {she is capable of doing so herself, but won't do it without me}
She always wets her pants {not puddles, just wet knickers} before being taken so she needs changing etc too. If we are home in an afternoon say, we'll go through a dozen pair of knickers.
She is starting nursery in September and, as my son is there now, I had the chance to get some one on one time with the teacher there to see how she would go, as it's been really worrying me.
The teacher reassured me that it's really common, and outlined how they deal with it, so I feel a lot better now.
Make an appointment with your son's teacher to go through it all and get advice and support from them before he starts.
The other thing might be worth looking into is urinary reflux. I had this myself as a child, so am getting my daughter tested just in case {although I believe her case to be behavioural, as outlined above, rather than physical.}
Usually that presents as UTIs, but frequent wetting / unable to 'hold' is a symptom.
The test is unintrusive and involves an ultrasound to check for scarring.
See if your GP thinks that is worthwhile.