Maybe see if you lfs will take him back for store credit, or even just give him to them to get him off your hands. Either that or if you know anyone in your area with a larger tank you might see if they want him.
As far as treating ich, most do so with hypo salinity. Do a search in the disease forum and there should be a thread that describes in detail how to do the treatment. You will need a quarentine tank though as you will have to lower your salinity to 1.009 which would kill you inverts. HTH.