Transfer of Title: Bought a Motorcycle from Son, Paid in Full Won't Surrender Title or Sign Off

My question involves a consumer law issue in the State of: California
Bought motorcycle from son, made payments and he said he filed the necessary paperwork but did not and refused to sign title and bill of sale so he remains owner and we have paid for it hanging it over our heads. I have proof of payment but some was in cash. I kept a ledger of payment and some are check and some are cash. What can we do? By the way these are sponsored bikes as he is a professional rider

Transfer of Title: Bought a Motorcycle from Son, Paid in Full Won't Surrender Title or Sign Off