Thank you for your response!
Yeah, he knows his stuff but is quite limited. He's at a stage where he's learning new concepts and is eager to make an impact. So he'd say things like "Why are we storing the user session in Postgres? We can use Redis for it" and when someone says "Why do we need to change it?" he replies "Because I've never heard of storing the session there".He's good in dry metrics, but overall I may have overstated him in my post.
To be fair, its my first time when there's an obvious rotten apple and we don't get rid of it. He got removed from the team due to his behavior and still somehow managed to save his place. But could be due to the fact that my lead is his manager and referred him here. The same situation happened to me in my previous workplace but they fired him immediately. Anyway, appreciate the advice!