Previously potty trained 3.5 year old having accidents

[Nutsycuckoo]

My 3.5 year old is constantly having accidents. He has been potty trained for almost a year now with no accidents but the past 6 weeks he is wetting his pants everytime he needs the loo. He refuses to sit on the toilet when i ask him to, having a full blown tantrum.
I have tried excessive praising when he (rarely) gets to the toilet in time, I have tried taking away a toy/privilege if he wets himself, neither works. The only thing that worked temporarily was telling him that he needs to stop wetting his pants if he wants to go to nursery (he is super excited about starting in September). This worked for 3 days, so I know he can do it, but we're back to constant accidents now.
I'm at my wits end now. Does anyone have any tips?

I think you need to stop making a big deal out of it. These things happen. Revert back to pull ups temporarily if need be. Stop punishing him for doing it as you will cause him to be anxious.

Is he getting distracted by fun stuff that he doesn't want to miss out on? My DS does this and though he rarely has accidents anymore we still have to remind him to go if he is wrapped up in what he is doing. We find making it clear that he isn't missing out by going to the loo and lots of praise works but also we make it clear on the rare occasions he does have an accident that it isn't okay nothing harsh just a sad tone / face when dealing with it and a reminder that it's yucky to go in his pants.

Don't go back to pull ups he'll just see it as a green light to not have to bother with the loo.

At 3.5 I would be making him help clean up accidents and put wet clothes in the wash. He will soon get bored of that. Don't put him in pull ups that's just giving in to the power struggle.