Thank you for the kind words everyone. I finally spoke with my brother this morning. Not a smart choice on my part to call him while I'm at work.
Anyway, Boo was almost 10 years old (as I figured). And he was diagnosed with lymphoma (or lymph something) which was in his lungs.
As I also figured, Boo was such a mellow, calm dog the cancer had pretty much taken over his body already by the time he was diagnosed.

Sunday morning they were getting ready to take him to the vet because he was very restless and pottying a lot. He went to the bridge before they were able to get him to the vet.

Just like Boo's "brother" before him, he made it easier on his humans and did not put them through the agony of making the decision to let them go.

We all take comfort that Boo and Digger are reunited once again.