make an agreement letter

I would like to check out is it possible if i would like to ask a lawyer to write and witnessed on an agreement letter (i do not know what it is called).

my circumstances is like this: Im pregannt right now but not geeting married with my partner. but he will bare the baby's raising money (by giving me a sum every mth). I would like to state in the letter that he cannot simply 'admit' the child as his without my permission in the future (though i planned to put his name in the birth cert) and few more conditions.

Could this be done? I just want a promise from him and witnessed by a lawyer, if he violates the conditions in the letter, can any law action to be taken in the future?

Please consult and roughly what would be the fees for doing so. Thank you