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It is possible to proceed as you state, but the problem to overcome is your son's Mortgage Lender. Before granting any Mortgage, they will want an assurance that no other party (ie you) to have any financial interest in the property.
Therefore the 65% would have to given as a gift to your son, and his Mortgage Lender will require a letter from you confirming it is a gift and that it is not repayable.
Your son's Mortgage could then proceed in the normal fashion and he could proceed with his purchase.
From your point of view, therefore, the risk is that you would have no financial interest in the property, so if your son got into financial trouble, for example, and the proeprty was repossessed, you would have no legal claim on your 65%.
I hope this assists you and sets out the legal position.