What would you do?

Okay so this is a going to be a bit long. But long story short I'm about to get married. My question is how do you guys split bills? My fiancé basically lives with me but he is still supporting his kids and his home. He does pay half of the grocery bill for my home every time we go grocery shopping and he gives me his half for his cell phone since the accounts in my name. What is fair as far as asking him to contribute to my household bills? Do you think it's fair if I ask him to pay one or two of the bills at my place? Water and cable? What would you do? He still has 3 more years of child support he pays as well just trying to be fair to everyone involved.