Or is DS?

This is a great idea...I'd even offer to pay for the Air BnB to help him to fully immerse himself in the "try before you buy" scenario for a good couple of weeks 🤣🤣🤣

Or when the train drivers are on strike.

He thinks he's going to commute two days a week when it's 100 miles away? That's crazy!

Why not tell him to book an Airbnb in the place where he wants to live for a month and commute in twice a week and see how he likes it. Preferably choose a time when it's raining

OP sometimes when buying a property its also good to have a view on the saleability of it for when you come to wanting to move on, so if your DS did go a head and buy a freehold flat it's always mindful that if he did want to sell it and a buyer needed a mortgage some mortgage lenders won't give a mortgage on a freehold flat as they know that the owner has no obligation to keep the property in good order.

Even if he expects to just move the loan who does he expect will pay for all the costs associated with moving? For surveys, solicitors, removal services etc? I think it’s entirely reasonable to say if you are going to move the loan you need to see how he’s going to fund the move. Presumably he won’t be able to show this so it should be an easy enough no.

I would tell him that the current loan is repayable when he sells the current flat.

If he wants you to extend him a new loan for the purchase of a new property then you will need to see full analysis of all of his income and projected outgoings so that you can be assured that the new investment is sound. It is what a mortgage lender would require, so see if you can find some worksheets from an online mortgage company or something to give him an idea of what he needs to include

He will also need to include a buffer for cost of living increases and maintenance/emergencies (like boiler breaking) and an estimate of moving expenses and fees, taxes and legal costs associated with selling and buying

Tell him that once he has done all of that work you will sit down with him and review it to see if you feel that loaning your money to him again would be a sound financial decision for you.

Just because the loan on this flat was a good idea doesnt mean he gets to just use the loan for whatever else he wants to going forward.

NT parents may be always wrong, but they are the ones with control of the bank accounts!

Ah and to answer your point, I'm guessing he is hell bent on this new property and won't have any rational discussion about it.

Would it work to ask neutral questions "How much does the train cost from Hastings to your place of work? How long did you say it would take again." and try and be fairly low key in your responses "So £50 a day that's £100 a week or £500 a month." Don't comment negatively about it "Gosh 2 hrs each way, what time will you need to leave in the morning?"

Disclaimer I'm no expert with Autism so I could be off the mark here, but is the trick to try to get him to see the drawbacks without directly pointing them out to him ?

Well there's your answer then. The rest of the money is needed for your retirement, so he can come up with whatever plan he wants, but you can't fund any more.

It sounds as if you are indirectly providing for his GF as well who is living rent free.

What is your financial situation like OP?

Can you sit him down and explain that you need x amount for retirement and therefore do not have any more money to give him as you also need to be fair to his DB.

It might be easier to justify why you can't give him the money, rather than exposing the flaws in his plan.

Tell him the money is locked away in anon-accessible investment account or similar, it sounds like he’s pretty clueless financially so I’m sure you can make some excuse. Or even if not, bottom line is you don’t owe him any of YOUR inheritance, he sounds dreadfully entitled if he thinks he should just be given what he wants. Time to cut the apron strings and stand up to him, if you don’t surely you can see you’re just encouraging him to make bad financial decisions because he’ll know there’s no real consequence and you will just bail him out and fund whatever lifestyle he wants.

Then going by the opinions on here your DS is being unreasonable, he shouldn't be moving until he's in a position to be able to fully support the move and travel/expenses that go with living in a larger property.

Maybe you can suggest to him that he goes and investigates ALL the associated costs of buying and selling property before he comes tapping you up for more money. He needs to know about removal costs, solicitors fees, stamp duty if applicable- has he got enough money to cover that? Can he afford the extra commuting costs? What about the GFs work?

I understand that you don't just want to say no. Asking questions rather than just giving out answers might get you a better result.

Unless he has access to your online banking he has zero idea how much money you have. You say unaffordable and impractical. He is incredibly lucky to have a parent that can assist so much.

I would say to him if he sells the flat, he pays back the loan.

he insists it's freehold or nothing.

that would be a nothing from me then.

Obviously your ds is unreasonable. Does he understand that moving costs money and there’s no more money available from you? Therefore if he wants to move it needs to be somewhere cheaper to take into account legal costs, stamp duty etc ?
As you suggested in a previous reply, would he respond to the logical route ? The 2.5 hour commute - is that in rush hour traffic ? If it’s public transport, is he used to how crowded and busy it may be (especially with changes, buses/trains being cancelled and delayed etc )? if he wants to move then changing jobs is a must. Are there jobs in these places that he wants to live in ?
Do you often go to his flat to help him out or check on him ? Does he understand that will stop if you are so far away ?

We haven't given it, it's repayable on sale,

Going on that if he sells his current property surely he must pay you back the loan money as agreed.

So how will he fund the next property he wants to buy?