"and has no liability since the 1099-R is already submitted to the IRS?"

The 1099-R is informational. It may have withholding on it, which isn't his Tax. It's a flat % as a mathematical construct by the IRS to get some tax amounts prepaid. Have you figured out if he gets a refund? Not filing means not getting your refund, if you are entitled to one. The entire 1040 is used for tax computation.

You already stated there are at least 3 sources of revenue to enter. Having the Distribution be any amount relative to any other amount has no bearing on anything. This is like asking if you need to include oranges when you are making a recipe that includes oranges and 4 other ingredients.

And there are something like 6 Modified AGI on the 1040 and its provisions, not just one thing that is the same across all provisions.