Hi funnykc. I've not had this problem (yet) because dd is only 2 but I was reading something on toilet training and it said that children often regress with toilet training when they got to school, and particularly boys. It is not uncommon for them to start refusing to use the toilet and/or wetting/poo-ing themselves and basically sitting in it.
From what I have read, it's something to do with the following: When they first go to school it is exciting but then, eventually, it dawns on them that this is it, they go every day and it's always going to be like this. Not only that, but it is a more rigid environment where the teacher is in charge and there is not so much flexibility for individuals to express themselves. This can lead to regression because a little boy wants to assert himself, be the leader, the boss. When he was at home with you, or even at playschool, it was less structured and you could reach an acceptable compromise between you.
So the toilet-training regression is a protest and an attempt to get attention and be in charge. So far, so obvious. He doesn't really know why he is doing it because he is just expressing himself, reacting to the stress he feels. His asking you to be like some other Mummies and take him/pick him up from school is because it would give him some reassurance, because he wants to be like his friends perhaps, and because he wants to see if you will do what he wants (has he got some say, can he assert himself).
Anyway, I'm not an expert but I don't think you have to give up work because of this as it is a phase that he will grow out of (just very difficult while it is happening). It is not entirely possible for him to understand at his age the importance of your working. That you are doing your best to provide for him, doing a good job and building a future for both of you. But if you could try and explain this to him all the same? That your going to work means that you can have a nice life and do some nice things. Ask him whether there is anything you can do to make things better for him (apart from giving up work). Offer him lots of reassurance and give him a say in some little things which make him feel that he is important. So, basically, don't react or get angry when he has an 'accident', show him a lot of attention, and give him loads of reassurance.
Don't worry. You're doing really well to be working (he will really appreciate this when he is older) and are a good parent too. There's always some compromise somewhere isn't there? The perfect mother would be working full time to bring in the money and set an example and she would also be at home full time to cope with every eventuality. It's not possible.
(BTW, the poo-regression-thing happened with the son of a friend of mine and she was a full-time stay at home married mother so it's not really to do with you working.)
Very long-winded as usual but hope it helps and that some others come along with more advice or actual experience of this. Also, perhaps post (if you haven't already) on the potty-training board.