It does seem like it still might be very open ended, and solve problems in a way that just makes for a boring RP.

Like... you might find yourself asking the question 'Why doesn't he just do XXX'? Like right now I can think of... transmute something into a new, unsunken boat, take that boat out to the marine ship and transmute it into sugar, make himself an impenetrable shield in combat etc.

I'd suggest including at the very least a size limitation (I.e. can only turn a pebble into something pebble sized), and that it only produces things he can picture in his mind, and so if he tries to make something too big, too complicated, or is just distracted then it's going to come out warped or broken.

Either that or go full Full Metal Alchemist and have it work on equivalent exchange.