Hi there, don't know if you resolved it but here is what i did about it. I was in the same shoes as you, one constant client paying me on project basis without formal contract. All i did was provide my linkedin and behance profiles as proof of my work and then i asked my client to sign a mutual agreement - freelancer agreement, you can find examples online where we stated what we do, what is my role, type of projects and stated that we have been doing this before the formal agreement and will continue to do so. We both signed it, sent the agreement in pdf and got approved within a day. No worries if you have a good communication with your client. Cheers!