By trying all sorts of tricks. Bribery might work for a bit, then change tack if it stops working.
So I get him to "try for a wee wee" on the potty and if he says no I might say, ok you just have to count up to ten or five or whatever on the potty and then if there's no wee coming out you can get up. If he really does need a wee it will come out in those ten seconds.
Or I leave him without anything on all day and then he doesn't have the comfort of a nappy so does go for the potty. That works better in summer though, it's a bit cold now.
We practise pulling knickers/pull ups up and down and seeing how fast he can race to the toilet and trying to beat his time.
Or if he's asking for some milk I'll say you can have some milk if you try on the potty.
Has your ds been able to see his daddy have a wee/have daddy teach him to stand, he might prefer to wee standing up?
He also likes looking at picture books on the potty. I do sometimes bring the potty out of the toilet if he absolutely won't leave a game.
If he's looking sneaky and shifty and I say, do you need a poo poo, and he then says "stop it" or "go away" I physically pick him up and put him on the potty, saying, "come on, on the potty, I'll help you!"
We have a step and seat thing on the upstairs toilet and a little £5 step stool from lidl in the downstairs one, he doesn't seem bothered about using the seat reducer we have (pottette) but he does like to have his feet on a stool rather than dangling.
I don't know about you but some potties don't seem big enough. Ds is not yet 3 and both he and his brother are slight yet their willies seemed to not go neatly into the potty - well the pottette one. I got a big moulded thing from France later on and that seems a bit more comfy for him.
A bit controversial is offering a chocolate coin for a successful poo or wee, which we do when I have them, and it was really helpful, and it doesn't have to be chocolate, ds really likes cashews and almonds so that works as an incentive too. (I supervise him eating them)