How soon is too soon to start dating after a divorce?

We've been dating for about three months. I can't say enough nice things about him. Extremely attentive, courteous, etc. But I sense distance when we're hanging out at his house (the one he shared with his wife). It's noticeably different at my house (more affectionate, etc.). I don't want to bring this up because I've been trying not to push him. My question is really geared towards men that have been in this situation, and their experiences. His marriage lasted for about three years. I trust him, but I'm afraid he still has feelings for the ex. For example, whenever she has a tragedy she calls him, but she asked for the divorce. What should I do? Also, I bring a lot of insecurity into the relationship from my last relationship, so I'm trying not to overreact. What are signs that a guy is really into a girl? Please help! By the way, we are both 30