How to help DS by a house

[House4DS]

DS is working far from home
Due to his age (19) he's finding it difficult to find a house to rent with a friend.
He has a 4 year job contract.

On his own he could borrow about 80k.
Houses are roughly double that.
I have no mortgage, own my home and have savings.

One idea is to buy together and rent out the second room to the friend under the rent-a-room scheme.
What are the pitfalls here? For both me and DS.
Capital gains tax?
DS pay his mortgage half, rent part pays mine with a top up, split any increase in value on sale?

I can't just give him the money as there are siblings to help in the future, but this could enable him to get an early start on the property ladder and solve landlords not wanting to rent to an 19 year old.

Help me see the wood for the trees please!

You might like to also look at options like Family Springboard Mortgage | No borrower deposit | Barclays (I was reading about it in passing yesterday, I think some other banks may have similar arrangements). It sounds like it was designed for people in your position. Good luck finding a good way to help him.

You could try looking into an Income Booster mortgage.
https://www.generationhome.com/income-booster

Basically, your income gets added to DS's income on the mortgage application in order to help increase his overall borrowing amount.
Everything would still be in his name etc, and he would still be liable to make the repayments, but you're down as an income booster and backup.