Being a little weak behind the shoulders is actually not the worst thing in the world...depending upon how far along the steer is.
When we bought our show steers (500# range), I always wanted one that was a little bit like that. It was a sign that they would typically stay loose in their spine and therefore be flexible in their movement.
So, what to do about it. It is really hard to give good advice without a picture of the steer, but I will try to imagine what you have and respond accordingly.
Get him fat, and get him hairy. As the steer approaches the point of being market ready, he should tighten up behind his shoulders. Part of it will be fat cover, and part of it will be him just tightening up as a whole. Whatever the overall finish doesn't fix, the hair should easily handle.
On a side note, I always enjoy hanging out on the wash rack at fat steer shows. So many of those really good ones are a little weak in their top. Again, that is what makes them move so well!