I think you are confusing the end of a discount or fixed period with a time that you need to renew the mortgage, where you don't need to actually renew, the mortgage would move onto the standard variable rate.
As you are currently on the mortgage with your ex, you should also be able to find out why he is unable to take your name off when he is replacing it with a guarantor of his father. Most likely it is due to affordability (and his lack of a job). Your best bet is to talk directly to your lender (A+L) about what your options are. It may be best to arrange an appointment with all three of you (the ex, and his dad) to speak to someone at the local branch (if this is possible). I'd also reccommend bringing someone to support you (a friend, family member).
You can also seek some advice from your local citizens advice bureau. (www.citzensadvice.org.uk)
It may be possible to take legal action to force a sale, but most likely this would take a long time and there is no guarantee that this would leave you in a better position.
HTH
Sparky.