Child support

Yes you are. SSI is a separate benefit which does not effect child support.

Yes, if I understand the question correctly. However, if the 18 year old is emancipated and while living on his won is eligible for SSI, then no. You would carry the burden of proving emancipation.

You are still required to pay child support until your son is 21 unless prior to 21 he: gets married, joins the armed forces or obtains employment where he is self sufficient. It is doubtful that receipt of SSI will equate to full time employment and self-sufficiency.