Hey Jen,
I always try to look at my motivation when i am trying to make a decision I'm not sure about. If you talk to your younger son, why would you want to do it? To give him a heads up and let things fall where they will or to see if you can get him to change? If it's just a heads up and if doing so isn't breaking your older son's confidence, i don't see the harm. But if you think you may be fixing something, you might want to check your codie-meter.
I also like the idea that not to decide is sometimes a decision. I've found when I listen to myself when I lean towards deciding not to decide, something more happens within a couple of days and everything plays out.
I suspect that your two sons will work it out one way or the other no matter what you do.
Prayers for you guys and sending another little kiss to Shelby Grace!