Is there a law on non-married division of real estate?

Normally he would not get any reimbursement from you since he is getting the use of the house. But I cannot say that would definitely occur.

He would be entitled to contribution from his co-owner only if the amount of the mortgage was greater than the fair rental value of the property.

There is indeed California law on the rights and obligations of co-owners of real property that apply to your fact situation. It has been too many years since I practiced real property law to answer you definitively but they are similar to how such situations are treated in dissolution cases (both are founded on equitable principles) but also different because of the unique aspects of the marital relationship and community property law. I would suggest a good real estate counsel, but if you want to look for yourself find a law library and look up the law on partition of co-owned real property.

You are joint tenants so jointly responsible other than from a contract point of view. Your argument is that he had the benefit of living there and did not pay you half of the rental value so you should not have to pay half of the mortgage. Also change it to tenants in common so you can will your share to whoever you want. With joint tenants if either of you dies, the other gets the whole property.

To avoid litigation you should pay him one-half of the mortgage payments for that period of time. If he took any of the interest as a tax deduction during that period of time, he should only get one-half of the principal portion of the mortgage payments.