We use Das every trip, as I have a special needs son (he's 25 now).
When you go to guest services, you just tell them you would like to get the Das service for your husband. The CM will ask what his issues are, you simply explain to them his difficulties, then they let you know if he can participate ( he should be fine). Then, they will bring out an ipad and take a picture of him, and ask how many members there are in his party. Then they will let you know how it works, and if you have any questions it's best to ask while you're there. Then you're done!
Will you be using magic bands or tickets? Either way, once you decide which ride you want a fastpass for first, go up to one of the CM's at the beginning of the attraction, and ask for a "return time". That's code for "we're using Das".
They'll pull out their ipad, scan one of the parties' magic bands, (or ticket, and anyone in the party can run and get the fastpass return time) ask who in the party will be riding (we usually say all) then they tell what time to return (they look at the current wait time and usually tell you to return in that much time - if it' 9:10 and the wait time is an hour, they'll give you a return time of 10:10.). If the ipad isn't working, they will give you paper fastpasses with the time written on them. When you return, make sure your husband is riding with you, he needs to scan his ticket first when everyone enters the fastpass line.
With a normal fastpass you have a one hour window to use it. With a Das fastpass, any time after your scheduled time is fine - however you cannot make another Das fastpass until the first one is either used or cancelled (you can go cancel it if you change your mind.) Then, you can send someone to make another one. It's pretty efficient and CM's are great if you have questions.
Let me know if you need any more info. - it looks complicated and can be confusing at first, but once you get the hang of it it's easy.