Your guy is either overloaded or he's doing it part-time whenever he gets around to it. If he's good at it, he's worth waiting on.
Sure, you could start looking for somebody else to do it. There are plenty of people out there doing it. Unfortunately, MOST of them aren't very good at it and don't have the proper gear or room. Some are outright frauds. You'd have to ask for samples from multiple MEs and compare them, and you still wouldn't know what the turnaround times are going to be until you put them to the test.
Personally, I do all my own mastering
if it's my own music, because I have no commercial ambitions. "Good enough" is good enough for me. But I won't master somebody else's work if it's going to be for sale. (OK, I'll admit to having done it, but only because there was no budget for "real" mastering.)