Kellyj, I really feel for your situation, but think that the reason your Dr is unwilling to sign this certificate is because technically it's not true (there's no medical reason why you can't return to work) and his professional license could be on the line. I don't think another Dr would help either.
Would it not be better to try and negotiate something with your employer? A month or two of unpaid leave, some part time hours, working from home etc?
it's tough being a new mum here, good luck x