The free dining offer is already out. It runs from November 9 to November 20, then from December 12 to December 23. They won't be adding dates. They have Thanksgiving week and Pop Warner week blocked out, and the week in between them. Used to, I think the week after Thanksgiving may have been included if you arrived on the right day, but not anymore.
Honestly as far as cost, I think it depends on where they stay on property. They now have to rent a car, so there is a few hundred dollars they otherwise would not have spent. They also now have to pay $17 a day for parking. If they don't mind a value, I don't think they would lose money. If they want something more deluxe, yeah it probably is going to be cheaper off property, but there is a lot of hassle that comes with off property. I grew up staying off property. I never once got to stay on property until I was an adult. Once I became an adult and stayed on property, I don't ever want to do anything else. If I had a family size or group size that made it difficult to stay on property, or if I planned to spend a good part of the vacation in places in the area other than Disney, I would, but for a standard Disney trip, no way.
One thing, I don't know if you are going to be there on DHS EMH night, but we have found that the Osborne lights are fantastic on EMH night. We wait a little while for the regular day park crowd to wander out and then hit it (usually about an hour after EMH starts), and usually it is not crowded and you can really wander and see things and enjoy it. Your parents won't be able to do that.
Disneyland staying off property is no big deal. There are a ton of places where you can just walk to the parks, and not even pay for parking. WDW is a whole different thing. Does he have any clue how far the offsite places are from the parks? Maybe showing him on a map would help? I would take the place he wants to stay and then do google map directions from that place to each of the parks to show him how far it is and how long it is really going to take. If he is thinking DL standards, he has no clue.
Also, if they stayed on property, then if he got tired of the parks, he could just leave your Mom there and catch transportation back to the resort, and she could do the same when she was ready. When we have been with my Mom or In-laws, there have been times when one of them decided to just grab transport and go back to the resort for awhile or in the evening when the rest of us wanted to stay later. Staying off property, he is either going to have to stay until she is good and ready to go, or make her leave before she is ready.
I predict some family squabbles. At 30 days- "No Dad, I'm sorry you can't get any FP+ times to match ours and ride with us, but we aren't changing ours". At WDW: No Dad, I'm sorry Mom can't come see the kids meet Anna and Elsa- you guys couldn't get a FP+, and everyone in our group that has one wants to be in there. No Dad, we don't have a car, so we can't bring Mom to your hotel later, you just have to stick around. No Dad- we have had these FP+ for 60 days, so we can't change our schedule and go to the show with you guys right now". No, Dad, we are entering the show with our FP+ and sitting in that section, we aren't going to wait with you guys and sit somewhere else, and we can't let you in where we are sitting. Dad, I'm sorry Mom has been mad at you the whole trip because you made her leave before she was ready, she had to deal with the hassle, and she didn't have FP+ to do things with us- you shouldn't have stayed off property.

Hopefully, it will still go smoothly if they stay off property and that your Mom is more easy going than mine who might have killed my father over that one

- but the way FP+ works, I think it is now very difficult when you have some people on and some people off. We usually have friends that meet us who live in the area, and it really messes everything up for us, because normally they get there and then the kids pick what they want to do and we all go get FPs- now we can't even book at the same time to be sure we can go together.