It is the statutory duty of the parents, to the extent of the parent’s ability, to support an adult child who is lacks the capacity to support themselves. If you are able to pay for the support for your child, then you are obligated to do so. The SSI benefits are for those who have no other financial source, or whose income is not enough to meet their basic needs. But since you are capacitated to support your son, then it is your duty to do so.