One thing I've found with ds, is that if someone ASKS him to do x, it is likely that his answer is "no". Then of course, he has to stick with his no. lol
If he is TOLD that such and such will happen, he is way more likely to just go along with it.
Most of the time, if I tell ds x will happen, and his response is still "no" I try to respect that. So, for sitting in the highchair, that would be his choice. Actually, most things are his choice.
Sitting in his carseat, well in some ways it's his choice. If its going to the grocery store, and he doesn't want to go, then he generally doesn't go. It's no big deal for dp to go later. Or for ds to stay with dp while I go.
If it's really not an option, say we are AT the grocery store and we need to go home, we just try to explain the reason, and make it as agreeable as possible to him. Maybe if we give him a toy, or say we'll play a game or sing a song with him while he's in his seat. But we tell him that it's not really an option as far as getting in the seat.
Another thing I've noticed is that if I say "you HAVE to..." it's a recipe for getting a "no". If I say "ok, time to get in your seat. We're going home." Just as a matter of fact, it is less likely to get "no" as a response.
As far as getting ds to STOP doing something, the very best way is to tell him what he CAN do instead. I keep the alternative as related to his original impulse as possible, that way he can see that I do care about what he was trying to do. We just need to find a way to make it agreeable to both of us.
So, if he's playing in his food, I might tell him "oh, lets not waste food. If you want to squish something, let's get the playdough out."
The important thing is that ds knows that I'm not trying to STOP him from doing what's fun, I'm trying to find an acceptable way for him to express that particular impulse.