How would a 17 month old little boy react to a little brother or sister?

The guy I am talking to has a 17 mth old and he is ADORABLE!!! Well I am going to hook up with this guy like next week and I'm not saying I'm gonna TRY to get pregnant but.. if it happen it happen :D hopefully it will happen lol But I just wonder how his son would take it?? Because his ex fiancee signed over the right to him so she is not even in the picture and this little boy needs a mommy!! And I would love nothing more than to be his mom lol so how would a little kid react to a little brother or sister??