Hi Greg, seems to me like they want to make you their jo-boy. I had a similar issue many years back, only this guy wanted me to abandon all my other clients and work exclusively for him. He said he would book me for the entire year, but there was no guarantee on the exact amount of work he would be able to give me. And in the meantime I would have lost all my existing customers.
Even though it might not start that way, it would only be a matter of time when they would consider that they own you, and would start demanding preferential treatment over your other customers. I would politely decline their retainer offer, just explain you have other clients who also need your attention. If they really like your work they’ll continue to hire you anyway. If they drop you that just proves that they’re really just looking for a slave. Hope this post helps, Adios Amigo