http://cssd.dc.gov/page/impact-ssi-and-ssdi-child-support
http://www.ssa.gov/survivorplan/ifyou4.htm
As a general rule, survivor benefits are to age 18 or 19 max, or to age 22, if dependent was disabled prior to age 22, but since he has MS, would have qualified for SSD or SSDI?
There is a possibility that you may have what type of benefit he is getting? With SSD or SSDI, I believe you have to work anywhere from 2 to 5 years to qualify and with his MS, am "guessing" he has never worked, or if so, may not have qualified for the credits?
My suggestion is to make an appointment with a social security lawyer and have son in law bring copies of all the letters that SSD has sent him in regards to what kind of payment he is getting and from what source. When he went from survivors, to, income/disability, they would have sent letters telling him how much he would receive, or, if they reduce any payments, they will send him a letter.
Either way, social security will go after him if owed, or can stop payments, reduce payments.