Well, basically the changes are there to ensure that he will always be able to act as Fay's food source since he is the only one the vampire can feed from. And since Fay will now live much longer than an ordinary human, Youou's life will be drawn out to match his.
Healing wise minor cuts will heal within a couple of minutes or so after blood is no longer being drawn. His blood will also replenish at an accelerated rate to make up for Fay snacking on it. On the downside his blood's ability to clot will decrease with the transformation for the same reason.
Most major injuries will also heal a bit faster than before but more on the lines of the doctors being happy that he is recovering so well, but not so much that they would be surprised by the progress. With the exception of blood loss where the healing process will be more accelerated since, again, Fai needs to be able to get his daily meal.
As for the immune system thing, I wouldn't say it would be absolutely impossible for him to get sick but it would take quite a bit since that would also affect his ability as Fay's "Game" (which I just realized is the official translation orz).
Most of these changes can happen gradually if that would be better, but the healing of minor cuts as well as being able to replenish his blood probably needs to be a bit more immediate, since it's going to be very much needed right away.
I have no idea how much details you need so I will just blubber on
Healing wise minor cuts will heal within a couple of minutes or so after blood is no longer being drawn. His blood will also replenish at an accelerated rate to make up for Fay snacking on it. On the downside his blood's ability to clot will decrease with the transformation for the same reason.
Most major injuries will also heal a bit faster than before but more on the lines of the doctors being happy that he is recovering so well, but not so much that they would be surprised by the progress. With the exception of blood loss where the healing process will be more accelerated since, again, Fai needs to be able to get his daily meal.
As for the immune system thing, I wouldn't say it would be absolutely impossible for him to get sick but it would take quite a bit since that would also affect his ability as Fay's "Game" (which I just realized is the official translation orz).
Most of these changes can happen gradually if that would be better, but the healing of minor cuts as well as being able to replenish his blood probably needs to be a bit more immediate, since it's going to be very much needed right away.