New Baby = Regression?

So I had my second baby nearly a month ago now and since she was born my May 2018 baby has stopped using the toilet. He wasn't fully potty trained but he hadn't done a poo in his nappy for months, I don't think he has done a single poo in the toilet since my daughter was born. The issue is he is now flooding his nappies/diapers because he bladder size is too much for them to contain. We use reusable cloth nappies and I've boosted them to the point I can't boost them anymore but he soaks straight through them in a single pee. He is dry on a night time so it isn't too horrible it's just the constantly wet clothes I'm struggling to keep on top of. He was pretty much potty trained before this and now I'm feeling crappy because I added a baby to the mix just for him to go backwards due to it. How do I get him to be willing to potty train again? Is it just a waiting game? He refuses to wear "big boy pants" now and asks specifically to wear a nappy. He also refuses to sit on the potty and will only sit on the toilet for about 10 seconds before he wants off 😭. I just don't understand how it all went so bad! Has anyone else been through this or something similar with their little one?

PFA