If Sam's medallion won't let him shapeshift properly, couldn't he still go to fullform by just leaving it at home?
Depends on how it's broken. It could be that the medallion just gave him the Unturned legacy spell again, rather than the controllable version it's supposed to create (or just didn't break the old spell at all), while still bonding to him so he couldn't use another. In that case taking it off wouldn't work, and even giving it up permanently might not improve anything (but would definitely make a real fix impossible). Hanging onto it and hoping he can fix it someday is his best option.