It is a great thing that his father is in recovery and wanting to reach out. However, the course of reconnecting does involve you. I would make a point to your fiancé that you want to be part of this. The father isn’t just developing a relationship with his son, he’s reconnecting to his son’s life and that includes you.
You are with this man for three years, and you obviously love him. If you were to leave now and the father bales in a month – what does that say about your relationship?
It sounds like the primary concern is that the father may be doing this in a manipulative way. That is very understandable and my strong suggestion is that whatever happens going forward goes slow. You want to be there for your fiancé so that we can talk about this process. And you want to be there to provide another view when it is necessary. I think if the relationship is going to flourish, then you need to talk about your concerns openly. Don’t be silent but don’t push your fiancé away. Remain skeptical. Your fiancé may be clouded by his father’s desire to connect and you can be there for support as well as be the voice of reason