frankly even though I don't like his old party, and the reason for kicking him is trash, but I still agree with lead to fire such a guy, because I would too.
What's the use of making party if I don't know your abilities/what you can do. you make plans with what you have at your disposal. This guy has been to dungeon and doesn't know what he can do? he is bad but can use such spells that can confuse him with top party members. he has been adventuring but doesn't know how important his speed spell is? are you kidding me.
This dude confidence is so low that it would make anyone depressed. I am introvert too and have met many introvert but we communicate with our team at least even if we are not going out of our way to talk to other peoples.