How do you make a child sit in a pushchair?

This is a very normal, though annoying, stage that most parents will encounter. Like all stages, it will pass.
Your son isn't doing this deliberately to annoy you: he's behaving like a normal two year old.
Establish some ground rules with him - do this before you go out, rather than getting annoyed with him once you're out. Point out the consequences, that if he'll sit in the pram, then you'll get to where you're going and start having fun much quicker. Maybe have a few special little toys that he can only play with when he's sitting in the pram. Make it clear that holding your hand when crossing the road is non-negotiable.
I know many parents have mixed feelings about using reins, but one new gadget you may consider is this child distance monitor, which can make a child feel more independent whilst still letting you know where they are.
If you can, make a point of taking some journeys at his pace, even if that means going two steps forward then three steps back (Look! there's a ladybird over there). Perhaps time your outings for when you know he's tired and may be more grateful for a ride. Accept that at this age, journeys will take longer than they would have if you were just pushing him along with no objections.
And if you're feeling brave, have you thought about ditching the pram for all but the longest journeys? Plenty of people do at two and a half. Before you know it he'll be off on a micro scooter and you'll be asking him to slow down.