How can I gently tell my boyfriend that he's a conversation HOG?

My boyfriend whom I love and adore is very smart, no brilliant. He has always inspired me to be better and smarter and I love him for his ability to articulate what he thinks and feels. But I notice that, majority of our conversations are not only dominated by him but they are usually "heavy" in topic. Meaning, he's always talking about the dismal outlook on the world, economical woes, business ventures etc. He's very serious-minded but sometimes I wish he would lighten up or just enjoy a silly conversation. It's draining, because he gets so intense in these convos and usually I may get 5 minutes of talk out of a 2 hour+ long conversation. I don't think he notices it, but I don't want to offend him by telling him basically to shut the hell up sometimes and lighten up. How can I express this to him, without him taking it as bad thing that this effects me wanting to always talk to him. Also, we live 10 hours apart currently. So, ALL we have are our phone convos. I'm sure if in person, we could do "other things" besides talk to p time.