Wait, how can he knot a beta?

Knotting doesn't necessarily need to be between an alpha and omega. All you need is an alpha dick and and a hole, at least as far as I know. Hope my answer helped. Mwehehe. (=・ω・=)

when an alpha knots someone, its just the base of their d*ck expanding inside the hole so that it won't come out

Yep, pretty much.