Can’t offer much in the way of advice, except to say do your homework before you take the plunge because, as you say, this is a can of worms. Friends of ours did this for their son and daughter in law who were first time buyers - both of whom lost their jobs within a few months of each other at the beginning of the first 2020 lockdown. They decided there was not much prospect of finding work, as they both worked in hospitality which has suffered badly. Ended up moving in with parents and the house went on the market. Luckily it sold quickly because mum and dad were responsible for the whole amount of monthly mortgage payments until the sale went through.
I think sometimes there’s a tendency to go ahead without proper consideration because it’s friends or family and the perception is that ‘it’ll probably never happen’ but if the pandemic should have taught us one thing, it’s that the unexpected can bite us in the bum without warning, so caution is needed I think. Could you afford to pay the whole monthly mortgage payment if something happened ? Maybe loaning them the money is a better idea, but could you afford to kiss that goodbye if, for some reason they couldn’t pay it back, or had to delay for a long time ?
Several years ago we were asked by good friends to be guarantors for a small loan they intended to pay back over about 5 years. It was from one of those companies who specialise in lending to people with less than brilliant credit histories, and they required security, which our friends didn’t have at the time. The monthly payments weren’t massive, but it would only have taken a moderate change in our finances to make things difficult. After reading through the paperwork and looking at not only the astronomical interest rate, but the penalties for late and missed payments we were horrified and had absolutely no compunction about saying no immediately. It put a strain on the friendship for a while, but not long afterwards they split up and the divorce was made all the more bitter by the fact that they had several loans to divide up and pay back. We were so glad we hadn’t got involved in the end, so don’t be afraid to say no - your son will have to learn to live within his means, so better to start now.