I think its either lack of leg strength or bend and snap. However, when he uses his arms to get off that block, he is effective. So he has some upper body strength.
He just can't get into a shoving match with the Guard or Center and not use his arms to get off the block.
He gets moved by double teams though not terribly. He has some quickness off the line. If he remembers to use his hands and arms, might be able to rush the passer.