Pants, underwear or nude?

I started potty training my 2.5 year old (speech and language delayed) son last Saturday. We did nude the first day, and he went to the toilet himself all day without me doing anything.

Day 2, we tried undies, and he peed in them every single time, so we tried loose pants, and he still peed in them.

Today, I tried undies again, same thing as last week.

So my question is, do I keep trying with underwear, trying with pants, or just keep him nude from the waist down for a few months/weeks until he’s no longer used to diapers?