How do SSI benefits work for children still living at home?

My husband has a son who is almost 18. He is supposed to pay child support until June of next year. Apparently his ex-wife is trying to have his son apply for disability benefits because he has stomach aches, or perhaps a disease called porphyria. It's all ridiculous and we think another way for her to receive money for this child after he turns 18.

If he is still living at home, but is legally an adult, does my husband still have to pay for him? What about when he leaves his mother's home and goes off to college? We do not feel there is anything worng with him, however, his ex-wife will be very upset when the child support runs out. This is the last child out of 3 that he has to pay for and he is finished!

Not to sound so happy, but she does not allow the kiddos to even speak to their father. Now that they are adults, they won't do it out of loyalty to her. She has manipulated BIG TIME to the point that 2 out of 3 won't have a thing to do with him. So needless to say, we are looking forward to this financial burden being lifted.