TEXTWALL INCOMING.
ok, first, he has little bismuth protrusions in his wrists and his feet and basically he can grow this protrusions to make multiple weapons, shields, armors or other constructs. he can use that also to destroy things from the inside by epanding (ex: he puts his finger inside a lock, start to grow more bismuth and destroys the door, he is tied up, he can grow bismuth until the rope snaps, he is inside a cell, he pust his hand between the bars and expands until he breaks the bars), he can also make little bismuth creatures, they are like like metal cubic worms that can perform very simple tasks such as all that i just said or leave them as traps, as centinels or simply lets them to grow so they can block the path to his enemies or even create basic structures that can be used as shelter or a rudimentary home. with enough time he can grow entire buildings.
his main weapon: ive seen that the weapons arent so much tied to the specific kind of gem as they are tied to the personality of the gem so id give him a pollaxe
en.wikipedia.org/wiki/Pollaxe, he would be very chivalrous and eager to help