Must i legally must give the baby my husbands last name -on the birth certificate-for child support or any reason?

That is a great question. There is no legal requirment, and having been married at the time of the pregnancy and birth makes him the presumed legal father, but I cannot think of any reason not to do that..and yes, it could help if there is a wrinkle of some kind. It is also the truth, and could raise questions for your child one day if blank. A lawyer or the department of revenue can help you get support ordered.