Fanon.

I was always confused if he was canon or not, cus i dont remeber him from the hp books/movies haha!

I'm also confused, is he supposed to be the brother of Tom Riddle Jr. or the son?

He is a fan-created character in a fanfiction, not sure which one. He is popularly "fancasted" as Benjamin Wadsworth. Mattheo is portrayed as either voldy's brother or son.

What I thought it was his dad what’s his dads name then

Mattheo riddle is the son of lord voldemort and belletrix. He is fan- created. His full name is Mattheo Marvelo Riddle. Benajmin wadsworth is the person that is fantacised as mattheo riddle. You can find a lot online about him.

Delphi's brother haha!

I was so confused in hearing the name lol. I just resorted to him being fan-made, and I was right. lol