Houston Astros legend Craig Biggio is still struggling to believe his son is a successful big leaguer ... telling TMZ Sports, "I'm still having a hard time putting it into words."
"It's incredible," the 53-year-old Hall of Famer says.
24-year-old Cavan Biggio is a rising superstar for the Toronto Blue Jays -- he was one of the team's top prospects in the minor leagues before he got called up just a few weeks ago.
Now, he's hitting in the heart of the Blue Jays' order ... and in his first-ever 125 MLB at-bats, he's got 6 home runs, 24 runs scored, 23 RBIs and 5 stolen bases.
Craig -- who's in NYC this weekend to watch Cavan play against the Yankees -- tells us it's a great feeling seeing his son's success ... and adds that the two talk ball from time-to-time after Cavan's games.
As for the advice Papa Biggio is shellin' out to Cavan?
"Just keep working hard!"
#Noted
By the way, Craig Biggio played for the Astros from 1988 to 2007 -- racking up more than 3,000 hits and 1,175 RBIs while making the All-Star team 7 times!
He was inducted into the Hall of Fame in 2015.