Seven shots to the back is excessive in my opinion, but in the legal sense as
@Tubey has mentioned a person can use the force that they feel appropriate...
... as long as they can justify it within the context of the event. Can he do that? I'm not sure, but there are cases when one or two rounds aren't enough.
I suspect that he will argue that he used x-rounds to ensure that the threat was removed, and it'll be up to the prosecutors to device - there isn't a set number!
But if we're talking about training of where to shoot, I'm not being facetious but it isn't like the Westerns or the films: you shoot to disable them and not to miss.
One stray round or one that passes through the arm etc (less mass.) could easily ricochet off the metal work or you could miss and kill or injure a bystander.
Therefore, you are trained to shoot the largest mass to reduce the chance of this. Does that mean he should have shot him? Personally, no... but he has.
As such, I'm simply explaining how or why he may have done what he has done. For me, there were far more suitable options than deciding to shoot him...
... where he was justified in doing so or not.