Hi Cassandra, I think that if he is actively sending clients your way and/or if you are somehow using your position at his firm to let his clients know what you are going to be doing then a referral system would be a fair thing to put in place. I don't think I'd want to lock myself in for the life of the client, although there are a few very successful VA companies who do provide ongoing referral payments, as long as that new client relationship is viable - so it's something you'll have to decide.
As a thank you to my clients for a successful referral I always let them know how much I enjoy working with the new client, what I like about them personally and about their project (so they can learn more about my own ideal client profile), and I send them 10% of the new client's first invoice, in cash. For clients who go out of their way to send numerous referrals on a regular basis I will often send additional gifts like office equipment, books, goodies, gift certificates to cafes in their neighborhood, etc.
Hope that helps!