I haven't had the same experience as yourself personally, but I think you should be encouraged greatly and now focus on the present time and building bridges for the future. We sometimes can never really know what prompts someone to change, but how amazing for you that he has. You may find it beneficial, if you felt that you needed to, to have some counselling sessions to discuss the past relationship with your dad and how it affected you and your mum. I am currently having counselling myself and I can say that if you feel able to talk then it is very beneficial and helps you to see things more clearly. I wish you and your family all the very best and hope that your relationship with your dad continues to grow well.
Best wishes, Fegans PSV