Help

[Amylouisexxxo]

So this time last year, I started potty training my 3yr old boy so he was using the toilet at preschool when he started.
Well it was a long rocky road of accidents every day for around 4-5months and then from then on the odd accident for around another 3-4 months and then I thought we had cracked it. Well it came to Christmas 2018 and they broke up for school, everything was fine untill they went back in the new year. He had his birthday just before Christmas so he is now 4. We'll ever since he's been back at preschool, we've had nothing but 💩 accidents every day in his pants but he's only doing it when he comes home and not at school. Nothing has changed at home or at school so I don't know what has started this off but I'm at my whits end, some of his pants aren't saveable so Iv had to throw lots away and buy him new ones!
If anyone has any advice, please help😩

Have you asked him why he is doing it? Could he be constipated?

Yes, I have asked him but he just shrugs his shoulders😩 he's definitely not constipated his poos are more on the softer side. I constantly been reminding him to go to the toilet when he's at home but even when Iv told him he'll just sit there 2minutes after and will do it 😩.
I don't know whether to get the potty out and have it down stairs again so he's got something close by rather then him having to run upstairs and being caught short etc