Concerning the conversation there's nothing said before, this is the first sentence in the conversation, but there're a couple of sentences talking about Alex who love a particular singer and Tom doesn't like him.
I expect the context to tell us whether the speaker is calling something weird or questioning whether the "he" being spoken about might be weird. Here is how I would punctuate depending on context/intended meaning:
Weird? He is a genius.
(Questioning whether he might be "weird")
Weird. He is a genius.
(Calling someone or something weird)
Weird he is a genius.
(Calling it weird that he is a genius)