He's not ready for the next step.
He will do it when he is ready.
We did that with my son too. My son was already about 3 years old.
We kept him naked on the bottom at home.
Then when HE was ready for underwear (I used Potty Scotty's with my son and love it), then HE was ready.
I went by his cues.
Don't rush it.
They can't control their body yet. Nor on cue.
Until my son was ready, he went out in diapers too.
No biggie.
They do NOT get 'confused.' I simply explained to my son, when his BODY is ready, then he uses underwear and can go out, without a diaper. It is in stages.
I just went by my son's cues and his biological, attainment.
Please remember, that even if a child IS potty trained, they DO have accidents. It is childhood.
I know of no child, that has not had accidents.
My daughter, even at 5-6 years old, had accidents.
Even at 7 years old, she had night time accidents.
Its no biggie.
It is BIOLOGICAL development of the brain/bladder/nerve development.
ALL of my daughter's Teachers, from Preschool to Kindergarten to 1st Grade, ALL said that kids these ages have accidents and even on field trips. It happens.
Night time dryness... is also something NOT biologically attained, until even 7 years old. Night time diapers are still used then. No biggie.
The child does not get confused.
My son is 4, almost 5 and he still wears diapers for night time and naps. I just explain that his body is not ready yet, to be dry when sleeping. But during the day he IS potty trained.
Normal.