Illegal box out

[Alex baxter]

I am 6'5" and normally play with people much smaller than me. I was playing with someone about 6'1", but more built than I am and whenever I tried to box him out or post up, he would stick his leg in between mine. This wouldn't actively trip me, but it prevented me from moving at all, or else I would be tripped. I've never seen someone use these tactics before and called him out on it, but he claimed it was legal. Is he allowed to do this, since he's not actively tripping me, even though he's restricting movement?