As a guarantor you can only be removed by consent of the Landlord. You can not remove yourself without consent or the agreement itself ends. Therefore at the end of 12 months and your son is on a periodic tenancy, if he signs a new agreement, you would be released.
To remove yourself from being a guarantor, you'll need to speak to the lender. The lender will need to review your application and make sure that you meet all of the required criteria. If you meet the criteria, the lender will release you from the guarantee.
If you are a guarantor and no longer wish to be, you must obtain the consent or agreement from the landlord before you will be released from your liabilities, which, if the rent is in arrears, the landlord is unlikely to agree to.