He's suddenly scared of the toilet

So ds2 (2.5) was completely day toilet trained using a potty. No accidents, was taking himself. He'd been doing so well for a few weeks that I decided to try him on the big toilet. First couple of days were fine there too. He's a shorty so even with a step he needed my help to get on but there wasn't any issues. But over the last few days we have started having issues with the toilet. He's scared to go. He's holding his wee until he's basically busting, most we are still getting in the toilet but we've had a few accidents. Poo's he holds onto until he has a belly ache and can't hold onto them anymore. So many times he tells me he needs to go to the toilet and when we get there he says no and jumps straight off saying he doesn't need to go anymore. I don't know what caused it. We've had a few issues with him holding his doodle down on the big toilet and we had a number of pees that have ended up on the floor and although I told him it doesn't matter, he got upset with it. I don't know whether that's set it all off. I'm worried him holding on is going to make him ill, and I am worried by the increase of accidents, I thought he was doing so well and we have gone backwards. I asked him whether he wants to use the potty again, but no he refuses to do that to. He wants to go on the big toilet, until he sits there and then he no longer wants to go anymore. We were giving him a couple of m&m's when he went but that doesn't even seem to be enough of an incentive anymore.

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!