That's what I was thinking. Might be easier just to go on Medicare.Gov and guesstimate the month. He thinks it was either 2001 or 2002. If the month is wrong for say 2001, go to the next month. Start with January and work through the months of the year. The hold times with calling Medicare are long this time of year. I tried calling earlier and was told a significant wait time.
I did this with a client. He told me he thought it was the summer of 2010. I started there for a few months and finished the year. Then started 2011 and it was that summer. It was actually pretty quick to do it.