1. Have you already set up an online SS account for each of you? That would tell you what each of you will receive - it is basically an online statement of your benefits which would help extrapolate your numbers. A starting point.
2. If I were you I would call SS, be prepared and focused, ask your questions then ask them to give you an appt for the local SS office and ask what documentation they will need to see - (maybe the SSDI is handled by a specialist?).
Tell them what you have in mind and if one person at SS can handle everything for you.
If you are lucky you will get a knowledgeable person who can answer your questions, make copious notes:) - then follow up at the local SS office.
... and NO, why would you even consider doing all this strictly by phone? This is important business.
These payments may one day be vitally important for your son - you need to do your best.
Personally, I would speak with an attorney who routinely handles SSDI before I made any final decisions. It is great that you've researched the facts and have come up with a plan, but this is one of those situations in life where professional advice is best and can save you money and trouble in years to come.
3. Bring everything with you to the SS office that might be remotely necessary for setting up these new payment scenarios. Be as prepared as humanly possible.
My own experience with SS has been mixed, both on the phone and in person. One office was fantastic, my situation was handled professionally and on the spot. In one visit I walked out with my head spinning - I'd receive my first payment in two weeks, incl some back pay and I would be receiving considerably more SS than I expected for the rest of my life.
It was the fact that I'd brought a ton of documentation, just in case, that made all the difference.
The next visit was at a different office and the worst experience of my life with any government office anywhere for anything.
YMMV - good luck!:)