I tried to see if he'd sponsored any specific pieces of legislation related to the matter, but unfortunately, I couldn't find any. However, I did find this:
Markey Leads Resolution Recognizing ME/CFS--May 15, 2018
It shows he's been advocating for us long before the pandemic made it "fashionable".
I'm not sure why I couldn't find any mention of him proposing legislation to fund ME research or education, but I have a wild guess.
Quick politics lesson for those of you across the pond: For a bill to become law, it must be approved by both houses of Congress (the Senate and House of Representatives), then get signed by the President.
But there's this obscure rule in the Constitution that all funding bills must start in the House. So maybe that's why he hasn't unveiled any legislation? A couple bills on Long Covid have been introduced in the House, but none have gotten passed and made it to the Senate.