I know deep down in my heart that it would be shameful to turn any family member down in time of need, but he has come to me/us for help.
My son has/had plans to get married in three weeks, but he is seriously reconsidering this whole marriage thing, and has decided to walk away from it. As a result of this decision, he has asked to stay at our home while he sorts out his life.
As his father, I live with a woman who is not his mother, and we live together in her home. My son is 29, I am 59 and my common law partner is 65. For whatever reason, his real mother turned down his request to move back home. We have the space ( bedroom, a full bathroom, etc.) for him, but having him move in while he sorts out his life will change out lives and how we live drastically. I am in no position to say "sure you can", but it is not my home legally and I am not sure at this point if my partner will be thrilled about the idea or will even want anyone else living in her home.
Morally, I would never shut the door on any family member who asked for help, and I want him to know that I would never turn him away in time of need.
I haven't spoken to my partner yet about his situation, but I am afraid that she will say "no", which will leave me in a very tough situation. I just can't say "no" to my son, for I love him deeply, understand why he is walking away from this marriage and would be devastated not to open my door for him in time of need. What do I do if my partner says "no" to me and to him? How could I ever forgive myself to turn down my son?
I really need some help on this one PLEASE!!

Just say no. If he can afford to get married, he can afford to find a place of his own to stay. If he is mature enough to propose marriage and get right down to the wire, he is old enough to continue on his own. He is old enough to start confronting his own dilemmas without a soft, easy, free cushion.
And you don't need to say that it's because it's not your home. Say no even if it were. He's not sick, injured, or on drugs. He's just not taking responsibility for his life.
If he wants your advice, let him visit for 2 days and nights, or go visit him. But I have a feeling that 'sort out his life' means he doesn't want the financial burden of marriage, house, children, and saving.

He can live where he is for now, and then move to his own place as soon as he can.

He does not need to take the "easy way out" But I agree, it sounds more like he will be a house guest that would never leave once he got there.

You just say no, tell him he is a adult and he has to deal with his responsibility.

Where is he living now that he would have to move? Is he living at the fiance's? Are you on good enough terms with your ex that you can ask why she turned him down? Talking to her may be enlightening and help give you direction on how to deal with your son. IF by chance, he does end up living with you write out something detailed like he WILL pay you xxx amount of dollars each month. He will clean up after himself and not sit around. He will work or continue looking for work. If he doesn't abide by your rules he is going to have to leave.
I have heard parents say they will charge their adult child xxx amount of dollars for staying there, BUT save most of it so they can give it to them when they go to move out toward security deposit and moving expenses.

It would be preferable for him to have his own place, but why isn't he in his own place now?

If he has already made up his mind, I presume his fiancé knows that he plans to walk away. Are they living together now while he waits for an answer from you?

Why did his mother turn him down, and is it possible for you to talk to her? It seems that you might be missing some information, and she could have a good reason for saying no.

Because of his age, he probably has a friend or two from the guest list that he could stay with until he gets his own place. Or, if you can, maybe tell him that you would be happy to help (if you can) with first/last on an apartment of his own.

And find out just how long he intends to stay. It doesn't have to be, or should be in my opinion, a stay without an end date. As he wants to 'sort out his life', that could take months, while simply finding a place of his own should be a much shorter stay.

I don't think it's all that unusual for adult children to return 'home' during times of distress in their lives- i.e. job loss, just out of university and not yet having a job, etc. but because a wedding is cancelled, surely he must have some resources of his own, financial and otherwise, at age 29?