Date of birth
Under 18 centre-half, who is a towering presence at the back, made several appearances for the reserves as an under-16 last season.
The defender, who was previously at Leeds United, played the majority of the youth team's games last season.
Mattie is the son of former Middlesbrough and Manchester City midfielder Jamie Pollock.
Was called up for the first team for a Checkatrade Trophy tie at Doncaster Rovers on 09.10.18.