Thanks for your question.
If you are purchasing with the assistance of a mortgage then you will have to disclose your plans to the lender. If you do not and allow him occupation then you will be in breach of the mortgage conditions. They will almost certainly consent to it but on the basis that it is let to your son and any other person occupying on the basis of short-term assured shorthold tenancies (ie. 6 months to a year) which can be renewed as appropriate.
They may also ask that your son and other occupiers sign a deed of consent/postponement which states that he does not have any rights or interests over the property and will vacate it in the event that it should be sold.
The difficulty is that shared ownership is social housing and the arrangement will likely include a restriction on sub-letting, were you to then sub-let you would be in breach of the agreement and this may entitle (in theory) the organisation to terminate the agreement and force the sale of the property. Make some discreet enquiries with the organisation you are considering contracting with to see if this is permitted.
You've got to do some more checking with the shared ownership scheme providers. You do not want to allow occupation and the have your breached enforced against you, which would result in considerable upheaval for your son.
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