If you are to have a share in the house a Trust Deed can be drawn up to record your respective shares. This would protect your interest both in respect of a claim by your son's wife or by any future partner or your son's creditors. If you are to become joint legal owners you will also be a party to and liable on the mortgage.

Similarly a loan could be secured against the property which would also give you protection in the same way. The practical difficulty is that a mortgage company will generally require that the deposit is porvided by your son from his own resources not a loan. Where parents are helping with the deposit they generally have to confirm that the money is a gift not a loan.

As the new house is being purchased after the separation and your son is not putting any of his own money from the marriage into the purchase, his wife would not generally have a claim on the property. It may reduce your son's claim to a share of the proceeds of sale of the matrimonial home (if they own a home together) as his housing needs have been met.

You cannot do anything yourself to protect the position if your son has a new partner and the money you are providing is a gift. If your son remarried and the marriage broke down in the future, his wife would have a claim on the property as the matrimonial home. Whether and how much she receives would depend on the length of the marriage, whether there are children and all other relevant financial circumstances.

If your son does not remarry, his partner does not acquire an interest in the property under current law simply because they are cohabiting. She wiuld need to show a substantial financial contribution or an agreement that she should acquire a share.

I hope this is of some help.