I believe that instead of owning the property as joint tenants they could own it as tenants in common, with his share being a larger percentage i.e. 60/40 thus protecting your investment in his favour. I expect someone else will be able to explain it in a more legal fashion. Either that or maybe you could have a charge on the property, although she would still benefit from its rise in value. I think you are being wise to think of this, as there are many tales of woe on here where people do not sort these things out and your son and you could really lose out if things went pear shaped.