I can relate to this exact situation - when I took on my first client I started out with her at exceptionally low rates and as my practice grew I found that I was making much more money working with other clients. But like you, this client was very important to me - and in fact, is the source of about 75% of new business for me on a regular basis. So not only did I grandfather her in but I have kept her rates lower than anyone else's for the last four years. We're still working together, she still sends me tons of referrals, and I'm still charging her about a third what I charge anyone else. Plus, I really, really like her
I would say, even if this client isn't a great source of referrals [yet] bring him in at the same or a slightly increased rate. Let him know you are charging him less than other clients and let him know why. Then teach him how to send you referrals. If he doesn't become a great cheerleader for you over the next few months, you may find it necessary to gradually increase his rates as your practice grows.