If you spent 300 and he spent 100, then combined 400 was spent, so you each owe 200. You paid 300 though, so he owes you $100. You can subtract that $100 from the rent you owed him.
His views seem to be correct. You want to add all expenses and divide by two. When you do this, one of you will most likely have spent more than the other. Whatever you have spent above the average can be subtracted from the rent check