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!"
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!