Financing flat for son

We are starting to plan for our adult son with high functioning autism to leave our home.  He is in his late 20s, and wants to live on his own, not in a flat-share.  He works and does not need continuous support, but will still need a bit of support with the practicalities of life. However, he refuses to allow any support worker to get involved, and whilst he has qualified for social care in the past, with the current cuts to services he probably wouldn't in future, so we want to find him a flat near his sister so that we can still support him for a while then hand over responsibility to his sister (who is happy to take it on).

Although our son is only earning the minimum wage, he is fortunate to have some savings which take him over the limit to qualify for housing benefit or council housing.  He could not afford to rent anything from his income, so would have to use up his savings on rent until it was all gone.  We think we could afford to buy a flat if we combine his savings, some money of ours and money from a trust set up for his benefit.  Is there anyone out there who has done this, who can tell us the pros and cons and any pitfalls we need to avoid?

Thanks in advance.