Thanks for your enquiry.
As your Dad succeeded to the Tenancy from your Mom, I am afraid that he is not now entitled to transfer the Tenancy to you or ask that your name be added to the Tenancy agreement.
You are therefore stuck with your wish to purchase the property. As it is, only your Dad is entitled to buy the house. If he can not get a Mortgage on his finances, the only thing you could try and do is see if your Dad can obtain a Mortgage with you standing as Guarantor (ie you become liable for the Mortgage payments if your Dad does not pay).
I can't guarantee you will be successful with this but it is worth enquiring!
I hope this answers your question, and if so, I would be grateful if you could leave positive feedback.