Oh Sarah!! I think you should try the direct approach and just ask your boss quietly if he has changed his mind about offering you the full time position as a result of your new circumstances. Even he would know that as he's offered it to you in writing (that email is binding, print it out if you haven't already), he can't say now that you're suddenly not suitable for the position because you're pregnant. Let him know that you were so thrilled that he thinks you are up to the job and that's why you were so emotional when he spoke to you. I'd also say that you're happy to talk to him further about maternity leave options, even though it's only early days with your pregnancy.
I hope that helps, and it's a good idea to get some advice not only from here but from a solicitor (I assume you have one ) Good luck, gorgeous!!