I think he already knows you talk to his parents. (Correct me if wrong)
I think you avoid getting in the middle as much as you can, and leave him to figure out his r/s with them, other than occasionally pushing him to be in touch... .and try to avoid confronting him with your contact of them, even though you don't keep it secret. (Also correct me if wrong)
This seems more significant than the 'normal' stuff and baggage between them, and you sound like you would feel really bad about keeping this quiet.
My suggestion... .pick one:
A: "H, your parents shared with me that they are making big life changes soon."
B: "H, your parents shared with me that your dad will be moving into a nursing home out of state soon."
You aren't burdened with holding a secret anymore.
And you aren't telling him anything about what to do. Leave that squarely in his court. It is his choice to talk to them or not.
He never even talks to them, let alone sees them, an uncrossable gulf of two miles isn't very different from an uncrossable gulf of 10,000 miles! So it may not matter to him or them.
And... .if he dysregulates... .it isn't your first rodeo

You get to brace yourself, AND set the starting time in a way that suits you!