We've not attempted proper potty training yet but what my mum told me worked for me and my brother was letting us run around in pants rather than naked and not change us straight away after we peed so that we could feel our pants were wet (which is generally not that pleasant). Apparently both of us learned pretty quickly :)
We potty trained our little one a few months ago and used the method from the ‘oh crap! Potty training’ book… We went fully naked for a couple days, then ‘commando’ (no underwear layer) for a full three weeks before introducing underwear. I think you’re right that adding underwear makes it feel like they’re wearing a nappy. Have you tried going commando under his shorts / trousers?
This is the same as my little boy, he’s just turned 2 and does amazing with no nappy he’ll run to the potty but as soon as he has pants on he’ll wee then run to the potty and sit on it 😣 my health visitor said to put a pad in his pants or some kitchen roll, we are yet to try as he dosent like the pad in his pants, good luck though xx