Give DS clear objectives, no more nappies, use potty/loo, keep pants clean and dry. You let him choose new pants for himself, and read lots of books about potty use. Get trained children to show him how it's done. Offer a reward, something he dearly wants, to motivate him. Think of lots of reasons why it is better to be trained and wear pants than have a nappy. Point out that everyone uses the toilet.
Don't ask, tell him it is time to go on the potty. Start with a structured timetable e.g. on getting up, after breakfast, mid morning, after lunch, mid afternoon, after tea, before bath, before bed. He can go at other times as well if needed. Remove all nappies so that you are not tempted to revert to them, but you can use pull-ups for night which you call bedtime pants. Always get him to try before going out or in the car.
Teach DS how to stand to wee, sometimes this can be a good motivator. Also outdoor wees at the woods, park etc. Appeal to his sense of vanity, say how grown-up he is and smart without a bulky nappy. When there have been a few successes, you could buy him new trousers/shorts of his choosing if that would please him. He has a reason to keep them clean.
Keep at it, and don't let DS think you will give in. Meet friends outdoors or at your home until you see some progress. You have to be pro-active, I think, because DS doesn't really know what to do. It maximises his chances of getting something in the potty, which you just won't get if you leave it to him, he will just do it in his pants because that's what he's always done.