I think the best way for your SIL to get rid of the liability he took on by continuing to store the stuff is to give your son notice in writing (certified letter would be best so he can prove he got it) with a specific date and time he will stop paying on the storage unit. In the letter, he needs to be very specific, stating something along the lines of “I will no longer continue paying for the storage unit on March 1, 2011 (for example). If you want to get your stuff out of the storage unit, I will be available the weekend before hand to allow you access to the unit to get your stuff. If I do not hear from you by February 25th, 2011 (for example), you are thereby giving me permission to close the storage unit and get rid of your stuff as I see fit.”
That way it shows a specific timeline for the unit to be closed, a timeframe when your son can get his stuff if he wants to, and also something that says what will happen if your son does not respond (that way he can’t say he didn’t know).
If your SIL feels the need to get a lawyer to look over the letter to make sure he is doing what he needs to do to get rid of the liability he picked up by keeping the stuff, that’s his choice. Honestly though, I think it’s best if you let the two of them handle it.