Can an my 18 year old son get welfare cash and how will it affect me as I am on Supplemental Security Income and food stamps and medical and he wants to move from dads to live with me?

He will not qualify for AFDC payments (Aid to Families with Dependent Children) since he is over the legal age limit. Since he is not the head of his own family/household, unless your son has been ruled to have some kind of disability he will not be eligible for any funds except perhaps unemployment. The fact that he lives with you will make no change (either up or down) in the payments you receive.