3.5 year old boy isn’t potty trained

We’ve tried potty training in the past. Bought his favorite character undies, tried letting him be naked, bribes, stickers, a sitting seat, a urinal, i don’t know what to do.

He did well for like 3 days then would refuse the potty.

He doesn’t even want to try his undies on anymore.

I don’t want to force him cause I’m sure it’ll backfire... but like? Something HAS to give.

Everyone thinks he’s like 5 so it’s super embarrassing when they notice he’s in diapers still.

He does have a speech delay, not sure if that could be a part of something bigger?

TIA