Thanks for your enquiry.
I am afraid that renting out the property would be in breach of the Help to Buy Charge and your Son's main Mortgage.
Therefore, the only thing your Son could do to retain the property would be to apply to his main Mortgage provider to convert the existing Mortgage into a Buy to Let Mortgage and pay off the Help to Buy Charge. I appreciate he may not have the funds to do this, and if so, he could try and re-mortgage for a higher mount to cover the existing Mortgage and the Help to Buy amount. However, as he only purchased last year, I doubt that the property has increased enough in value to do this, as you normally need a 20% Deposit for a Buy to Let Mortgage.
He could of course just go ahead and rent out the property and leave matters as they are, but I can't really suggest this, as he would be breaching the terms of the Mortgage and the Help to Buy Charge.
Sorry this is not the answer you were looking for, but it sets out the legal position.
Hi, can I assist any further? If not, I would be grateful if you could rate my answer. Thanks Al