Legal Question in Real Estate Law in California

If you are asking whether you have a right to occupy property that you co-own, the answer is yes.

I have to sorta guess what you mean by "I own a house and I have a second buyer," but I suppose you mean you co-own the house. Are you a tenant in common or a joint tenant? Is your housemate the son of the "second buyer" as well? What were the original ownership percentages, 50-50 or something else? Also, when a co-owner pays more than his/her share of the mortgage principal, this can raise the co-owner's percentage ownership above what's shown on the purchase documents. I suggest that you re-ask your question with more details given and/or seek a free initial conference with a local real-estate lawyer.