Assuming that you're only making FP+ reservations for your immediate family, I'd just make the best plan for him, then tell them that these are the reservations you made and invite them to try to grab the same ones. You could also offer times that you'll be available to meet up of they don't want to get the same FP+.
Then go to GS once you're in the park and link the tickets for whoever you'll be with that day. Even if for some reason you can't or don't want to link everyone, they could still get in the standby queue when you get a return time for your son and you should still end up riding at around the same time. On many rides a party of 7 would be spread out anyway, so 3&4 isn't that inconvenient. The only thing that might be
an issue would be if someone else in the party also has a DAS or needs a WC return time. In those cases a person's ticket can only be linked to one DAS and only one return time can be active. But if it's just the one DAS then GS can add people as they join the party.
If at all possible, I would suggest having all 7 in your party with at the time DAS is requested. That way, you will only have to go once to Guest Relations. If thats not possible, you will have to go back to Guest Relations to add the new members as they show up, as the poster above and others mentioned. Adding a few at a time would involve more Guest Relations visits.
The usual number for DAS is 6, which is 5 plus the person with a disability. They were able to make exceptions for larger groups, with a lead or supervisor's approval. As far as I know, this has not changed. A group of 7 who are all family members
probably will not be an issue (we can't make promises though since only Guest Relations CMs at the park can issue DAS). If your group was larger than 8 or 9, I would expect an exception not to be made.
Since linking DAS to tickets or Magicbands is so new at WDW, we don't know whether all members of your party need to be there to get linked or if they can just give you their Magicbands/tickets. In the past, if your group was bigger than 6, all members needed to be physically present in order to get an exception. I have not heard anything specific since DAS started being linked to Magicbands, but expect that still will be the case, so expect all will need to be there.
If more than one person in your group needs a DAS, you CAN have all members of your group linked to both DAS at WDW. This is according to reports from people who have gotten DAS since the linking began.
Each DAS can only have one active DAS Return Time at a time. When you get the DAS Return Time assigned, you specify which members of your group should be assigned to that DAS Return Time. Even though people may be linked to more than one DAS, they can only be assigned to one DAS Return Time at a time.