I'm confused, you now say he paid more, but you said he paid 50% of one rig in your first post.
The only fair distribution is that he gets his share only from the rig he participated in paying for, not the other one you paid fully yourself.
If he paid 60% of the other rig, he gets 60% of the bitcoins mined by that rig (use a different pool if it makes computations easier) minus 60% of the electricity cost of that rig (assuming you're the one paying the bill.) Buy a Kill-A-Watt to check how much power the rig uses.
If the rig is responsible for 10% of your home, electricity cost is 10% of the fixed bill plus 10% of every KW range.
You probably need to do the computation only once, and set his electricity bill as a fixed charge.
You can both decide whether his electricity bill is either paid in cash or deduced from his mining rewards at the current change rate at the end of each period.

You obviously need his approval for this change, since you already agreed on 25% of both rigs' mining.