Why has he invited his mum to stay over your birthday - has he done it because he forgot it was your birthday, because he genuinely thought you would like it, because she asked to come that weekend, something else? Does he realise that you don't get on or does he just assume that you do and he thinks that he has done something nice... Think before we know how to react to what he has done for you we need to understand why he has done what he has.
Could you tell him that you are not throwing him a party but instead taking a leaf out of his book and inviting your mother to stay for a few days over his 40th? Even if you have no intention of doing this, definitely think you should tell him that you are going to
What would you have liked for your birthday - would you have liked a party too or are you holding out for a party on your 40th/already had a big party previously (sorry, don't know if you are older or younger than your dp)?
If you would have liked a party for your birthday (or for a future 40th etc) and you want him to organise it then I guess you have to do one for him.
However, I do think you need to decide what you want to do for your birthday - am guessing that your MIL is not part of it and sort that out separately making sure that MIL is well out of it.
Would your dp be prepared for you to go away on a girly trip and have a big party for your big birthday? If he would and your finances allow for it then I would use it as an opportunity to educate him as to what you want and don't want but if not then just do what you think is reasonable.
And why is he not able to organise his own party? Hardly going to be a surprise party if he has asked you to organise it... Say you are prepared to pitch in and design an invite/make a cake/whatever you fancy doing but that if he wants it, then to make sure that he gets the party he wants, he needs to do it and you will help out - otherwise if the slightest little thing goes wrong it will all be your fault. Is he used to having his mum organise this sort of thing for him in the past and sees it as a show of love for him or something like that?
Sorry, lots of random questions but from what you have written I think there are loads of unanswered questions jumping out! Hope you get a chance to mull over everything and at least get your MIL un-invited and the birthday that you want!