A: You should speak with the state office of Family Services or Child Support Services. I am not saying you need either of their help, but they are familiar with the specific processes of amending and updating birth certificates and they should be able to steer you in the right direction.
A: What are you worried about? When your son turns 18 the non-custodial parent no longer has to pay support to the custodial parent. Are you custodial or non-custodial?
In a situation where a child has disabilities support may be continued further than that. I'd need more information from you in order to determine what you might be worrying about.