Yeah id be carefull with that aswell
Say if he just did turn up and asked to be put on, and you wernt to sure, but put him on anyways, it will give him equal parental rights over the baby. So say if something happened to you, and your plan is for the baby to go to your parents or whoever, He will effectively just be able to take them, even if you make a will, stating that X is to have the baby, it will most likley not be taken into consideration as parental rights always rate alot higher.
Of course that only applys if you dont want him to have them if something happens to you.
And i also think it gives him equal say in things like schooling ect ect, though dont take me up on that for deffinate. It all depends on how much control you want him to have.
If you dont want too put him on, but think he will just show up, then dont tell him when you are going to register them, It sounds horrible, but its what im having to do, not that i think he would show up anyways lol