Seems if you reach up to block the knee he can just lock that up

There's a traditional escape for headlocks: Short ( sucker ) Drag. When someone grabs your own arm to headlock you, you simply use that "same arm" he's using to choke ( headlock ) you, and hold the outside/back of his elbow, and simultaneously side-step towards his back foot. In other words, you'll end up facing the "same direction" if completed correctly. Also: Dip your head downward while pulling the elbow. The sidestepping opens up the 'headlock" hold, via ANGLE moving via "T" ( perpendicular).

show us your moves