Demand his #7 for Upton as you're giving him a top 15player out of a pool of 50already kept and he would more than likely be selected by 1/2 in your redraft pool. If he wants Upton this is how he gets him or else going to someone else who picks ahead of him.
If I were you I'd be Keeping Upton and letting go Castro. There are so many more players surrounding Upton now with true talent vs. who Castro is surrounded with. SS has many callups coming that if your pick doesn't pan out they just may turn out better options.
You really want to Keep Castro then counter the 7 being a must as well as the 14 for Headley. If he refuses, say 7,12,and 14 for Upton,Headley and your 11pick.
The other owner needs to realize he'll be able to keep Upton going forward, so if he wants him, he's gotta pay the price. Otherwise Upton won't be there for his 7th pick overall. You hold the cards here bro.