Or is DS?

If the money is (thought of as) yours which was used to help buy a particular property, and that seems likely given the loan is documented as being repayable on sale, then I think you really do just have to say no. You will not be investing your money in a freehold property which needs work miles from where any of you live/work. That is not a good investment for you.

I also have ASD adult children and it's important to recognise they might see situations in a particular way but that doesn't mean you have to go along with everything they want. I would not get too bogged down in the detail of why you don't think it's a good move for your son, just that it's not a good investment for you and you will not be contributing anything towards it - not the original amount which needs to be repaid and certainly not anymore.

It's ok to say no. No matter how old they are your children's love and place in your lives should not be dependent on how much money you give them.

You're going to have to be brutal and just say you will have the loan back, and won't be giving him anything to fund the freehold house. Bank are unlikely to lend him the money, job done.

Can’t you just say he’s an adult and so he needs to do this himself and that buying him a property was a one time thing, not something you’re able to keep doing every time he changes his mind? How old is he? How long as he been in his current flat? Does he expect you to buy him somewhere new every time he decides he wants to move? At some point he needs to learn to stand on his own two feet.

I dont get why you can't just say no? Why all this round the houses?

If you’re concerned, just say no. Explain that you were happy to loan him the money for his current flat but that you’re not able to do that again and if he wishes to move he will need to do so with a mortgage like any other adult. He sounds very entitled if he thinks you should just buy him a house now he’s bored of his flat.

A train fare of 100 miles is easily going to be £500 a month. Has he factored in the extra cost?

If he can’t afford to buy anything without your help then obviously it’s fine to refuse, you’ve already bought him a flat, I don’t know why he would expect to get another handout. Just tell him you’re not in a position to give him any more money, that’s completely reasonable. If he wants to move then he needs to stand on his own two feet and earn the money himself, that’s a completely reasonable expectation of an adult.

Why isn't it as simple as saying "no"? He's an adult and can do what he likes, including selling a flat that's presumably registered in his name. That doesn't mean you have to provide him with any further funding.

So just say no. No need to justify your decision. You've given him a loan and that's all you have to give.

So does he own his current flat? Sorry that wasn’t really clear in your OP

An interest free loan from the OP plus an inheritance from his grandmother.

You have given the money. It is no longer yours. You say interest free loan, I assume you have an agreed repayment rate as long as he is meeting that then you have very little say.

If you don't have an agreed repayment rate I suggest you start so that you don't get annoyed by his plan as you have a smaller amount invested and he has less cash to waste on grand plans.

So does he own his current flat? Sorry that wasn’t really clear in your OP, to confirm are you saying he currently owns a flat but wants to sell it and buy a freehold one 100+ miles away? Does he pay a mortgage on his current flat? Would he be able to afford to move and pay stamp duty and solicitors fees etc and new mortgage costs etc? To be honest if he can’t even manage a simple budget it sounds like managing to sell his current flat and move is a complete pipe dream anyway, I would just stay out of it and let him explore the option as it sounds like he won’t be able to make it go anywhere anyway.

Sorry but you're really enabling him. You're not helping with how to cope with adult life once you're gone one day.

Have you actually asked him how he intends to pay for all of this. Or does he think that as you got one loan you'll be getting another for him.

But it is?! The ASD is a red herring. If he didn't have it would you opening your purse at every beck and call? You're not teaching him how to be a grown up - he needs to learn he can't just snap his fingers and the bank of mum and dad will make everything better.

Okay, sorry that wasn’t clear in your original post. To be honest in that case it doesn’t sound like he is responsible enough to be buying a flat, if he can’t manage a simple budget or understand finances in this way it sounds like owning a property could be a disaster.

Apologies I mis read. Just tell him no sorry he's fine where he is, he doesn't need to move and clearly can't afford to.

Honestly he doesn't sound mature enough for you to make this massive investment. I'd let him rent for a while.