Well i always knew. Ever since 11 came around he got angry. At some point mike tells will "it's not my fault you Don't like girls" or something like that. They moved to lenora and 11 feels will is in love, she thinks he's got a girlfriend, he spends too much time drawing. Then he shows mike one of his drawings, the masterpiece. He took it with him to give it to mike. Tells mike "you're the heart", because he noticed Mike's feelings towards 11, he tells mike that eleven asked for it. He turns around and cries.
He cried because he treated her bad back in lenora because of jealousy, mike wrote her and called her but not him. I think that this was obvious since season 1.
