Must be the day for toileting posts - I've just replied to somebody else's post re: poos.
Our DS2, also 2.5 went really well on the toilet at first and then decided he didn't want to use it anymore, which resulted in lots of accidents on the floor. He hasn't done a poo on the toilet yet and waits til he's in a pull up for sleeps.
We started off with a seat that has a step with handles and he sat down to wee. We had to get him to point his willy downwards, otherwise the floor got wet. When he decided he didn't like the toilet anymore, we got a step and he started doing 'big boy stand up wees' like his older brother. This has worked well (so not sure if the opposite might help your DS and swapping to a seated position???) The other thing that I found was useful, was letting him do an 'outside' wee in the backyard and getting him to tell me when he needed to go. It takes the pressure off trying to get to a toilet on time and he finds it really funny. Just trying to get him confident again about telling me when he needs to go and not worrying about making a mess. I've also made it a bit of a game (let's see who can run the fastest to the toilet) and we've been using a sticker chart.
Also, if we're about to do something/go somewhere, we say 'first we'll go wee, before we can go to the park etc'. Lots of high fives, and applause when he uses the toilet. But boy it's a one step forward, two steps backwards process.
Hope you get some wins soon!