I haven't seen the 2016 DP packages and prices yet. What's likely to happen is you can't add the DP until the 2016 info is announced. I don't believe they'll let you prepay at the 2015 price and lock it in. The only protection you'd have is if they raise the price on the 2016 plan later, you'd be locked in but that's unlikely to happen for Jan, 16. It has happened at least once where they raised the DP price mid year but didn't raise the DVC price to match, I'm guessing due to contractual issues with DVC.
You pay when you add it. You can potentially make changes up until 48 hrs before but when renting it's more complicated to get this done. If you haven't already, I'd contact the person you rented from and let them know you're thinking of adding the DDP later and how would that work for them. You'll either have to pay them and they would pay Disney or you'll have to do a conference call with you, the renter and Disney. Canceling a person later would be a hassle and would add risk. If things otherwise fall into place, I'd add the plan for the group at around a month out. If you're still uncertain as to whether the grandson will go or not, I'd run the scenarios with him but not being listed on the DP and having the DP for him if he doesn't go. If he's 9 or under or it's a short trip, having the extra credits might be OK. Paying OOP with you having the DP without him listed isn't likely to be a major issue. Technically everyone is supposed to be listed but in this situation you may not have that option.
