Cody Bellinger is having such an insane rookie year with the Dodgers, his own father can barely believe the numbers ... but tells TMZ Sports, "It's really fun to watch."

We spoke with ex-MLB player Clay Bellinger -- who won 3 World Series during his pro baseball run -- and he tells us he's incredibly proud of his boy. 

FYI, Cody has already blasted 18 home runs -- more than anyone else in the league since he was called up on April 25th. He's currently tied for 5th overall in HRs. 

Ya gotta hear Clay gush about Cody's meteoric rise. It's awesome. Plus, Clay reveals the Dodgers legend who's really gone out of his way to help the kid.