The off-axis suggestion will help a lot.
I find the whole "tell him he sucks and move on" thing to be funny. I got paid to record people for many years, and for a time sort of tried that approach. Let me tell you man, I made a LOT more money, and dedicated customers when I worked my ass off to try to make them sound as best as they could. The whole "tell them they suck thing" tended to not put food on the table!
Really, as the recording engineer, is it my JOB to tell them they suck? No. I am there to provide a SERVICE to the CUSTOMER. Keeping my opinions to myself, and not telling customers that they "suck" are two great ways to stay in business! You can be as fucking altruistic as you want about the ultimate talent level of the artist you are working with, but in real life, you WILL offend them, even if they ask for your honest opinion.
Yes, I have had cases where I COULD share my honest opinion with the artist and not offend them. But, these were very rare, and a good working relationship had to be established. Also, I had to know for CERTAIN that what I was sharing would serve a GOOD interest in the project. Let's face it, there are just times when you are hearing the best you are going to get from them, and there are no other options, and I have to go to work to make the best of it! How many times have I "fixed" the drummers mistake? Let's see:
Band just paid $1000 for setup, getting sounds, and 10 songs worth of drum takes. There are a few mistakes the drummer made. There is NO WAY he is going to get it right AND have the sound retain some excitement.
a) I tell him the takes suck and make him do it again, thus, taking away money in the budget that most likely the SINGER is going to need!
b) Tell him he sucks and the band breaks up and they go away remembering me as the guy that told their drummer that he sucks that led to the breakup of the band.
c) Fix the fucking drum tracks and move the fuck on!
The "pro" is going to do c every single time!
One trick to deal with this singer is to make him sing "up" to a mic that is above his forehead. This will force him into a better posture for singing, which will also get him out of his throat somewhat. Also, it is quite easy to control the proximity effect of the mic. It also picks up more of the nasal tones of the singer, which tend to help clarity.