If he's not your spouse or partner, don't mention it. Just fill in the childs school or doctor on the back, and send it back, freepost.
Child benefit is paid to the mother of the child (the last bastion of old-fashioned state-sponsered sexism).
As a foreign national, you will get this form every 3 months like clockwork. Failure to return the form will result in the benefit being stopped.
My wife has been getting this form like clockwork since our daughter was born last year. She too is a foreign national (and therefore, according to the Child Benefit section, a thieving scammer who must be checked up on every 3 months). It makes no difference that I'm Irish. Only the mother's nationality is considered.
Anyway, I'm sure all these forms make a nice pile of work for the Child Benefit section to keep Brendan Howlin and his budgetary knife away.