Unfortunately, sometimes you have to let your client make mistakes. All you can do is offer your opinion based on sound reasoning, and then if he still decides to proceed, that is his prerogative. You can then later charge him for your time to convert his MT blog to WP. ;-P
My husband used to drive a cab (what psych. major hasn't?). He taught a novice cabbie something and there is a close analogy to your situation.
A customer from out of town gets into the guy's cab. He has a map which he hands to the driver with the route already planned out. The cabbie, familiar with the city, tells the passenger he can get him there faster (and cheaper) going another way (the cabbie's way).
Of course, at the end of the ride, the customer bellyaches about the fare. The lesson? The customer had his route mapped out for him. Even if that way costs more and takes longer, the customer will never be sure that any way except THAT way is going to be faste or cheaper; his mind is already made up. Best thing to do is color by numbers and collect your fare at the end.
See what I mean? Even if you convince him to switch to WP, his mind may very well already be made up. He has to be convinced to change it on his own terms and this may not happen unless he becomes frustrated with MT for whatever reason. But the decision has to be his, unfortunately.