A lesson horse I used to ride would do that all the time because she didn't like being in the corssties so we would hold her in the aisle or tack her up in her stall
My mare did this a couple of times during last winter when I first got her. The reason behind it was that she wanted to go to the field with the yard owner's horses, so she reared and got free. I didn't blame her - she was hardly out during winter because the yard owner was really fussy with her fields.
Try and find out the reason behind why he's trying to get free. Maybe he wants to go out? Perhaps he's anxious and wants to be with another horse, so he fights to get to one? Maybe he's uncomfortable in the head collar he's in?
Worst thing you can do is shorten the lead rope and restrain his head. Lengthen it to ensure he can freely move his head. Best thing you can do is find out why he's trying to free himself from being tied up and solve the problem from there.
Horses that do it once tend to do it again. Don't Risk it like I did and just get a blocker tie ring. My mare did it a week ago and split her head open and fractured her skull
Something might have spooked him while he was tied up. For now take caution when tying him up (that's why we just slip the lead rope around the post if we're in the area still and he stays still because he's used to that)
Something could have spook him or maybe he had a past experience that scared him