Quinn, to my mind, you ought to take it upon yourself to find out more about MS and possible treatments, then be brave and contact your step son's mother. You have made the first step by writing here so you must be a caring person. More women than men with MS end up without a partner so to end up without a son as well would be too much for her so both need help. Many people might say that you and your husband need to apply for total care of the son, since his mother often isn't capable, but that would be totally wrong: imagine if you were the unfortunate woman left alone and helpless whilst her son was enjoying his life with his new family.
You don't say in which country you live, but you should be able to find the national MS society easily enough, then they will direct you to the local society. If you are in the UK, there are many local MS therapy centres who have minibuses to collect and deliver people who need them. I guess many other countries have similar. Who knows you might well end up very good friends with you step-son's mother and she will be endlessly grateful.
An Itinerary in Light and Shadow Completed Dr Charles Stratton / Dr David Wheldon abx regime for aggressive secondary progressive MS in June 2007, after four years. Still improving with no relapses since starting. Can't run but can paint all day.