It is stress/anxiety... a new baby in the home.
Just comfort him.
He is young.
Just to let you know: ALL of my Daughter's Teachers, from Preschool, Kindergarten AND 1st grade, ALL SAID that kids this ages have accidents. NO biggie. They just have you bring in extra clothing for your kid, to change into.
Even my daughter's school Health Room, has Extra clothing/clean underwear for the kids at school who have accidents.
It is childhood.
Don't stress him or punish him for it.
It will pass.
When I had my 2nd child, my fully toilet trained daughter, had accidents. We did not scold her nor her Teacher. We comforted her. We did not pressure her. We KNEW it would pass. We talked with her and comforted her. We did not make her feel like she was inadequate or that something was wrong, with her.
I told her, that I understand. Its okay.
I told her, I KNOW she is a little girl, not an adult.
And then, it passes and she stopped having accidents.
Also, kids this age "think" they can hold their bladder longer. But they can't. Keep in mind that kids this age, even if toilet trained, CANNOT have total 100% control over their biological urges.
Night-time dryness for example, is not something that is BIOLOGICALLY attained, until even 7 years old. It is about the body's brain/bladder/nerve development. And yes, even at this age.
all the best,
Susan