When he’s moving to just a single bed, any good quality mattress should be ok. Depending on continence you may want to buy protective bedding

My 2.5yr old has just moved into a big bed now. She is sleeping on a normal single bed mattress with springs that my 4yr old was sleeping on in a bunk bed. I think they can sleep on any single bed mattress. You should buy one that will last a long time with springs not a foam one.

Innerspring is an absolute must be check your measurements as some can be thicker and therefore more difficult for him to be able to climb up and down.