If you decide to make the change send him this:

Rimshot
Sorry for the late post. 
Because your current drummer has been with you for some time and you do risk the chance of losing both drummers if you let the old one go and new just doesn't work out for some reason, you might want to consider this:
Have a serious talk with your current drummer and let him know that he is risking being replaced unless he comes around and takes his role more seriously.  Tell the potential new drummer that is what you are doing.  
Everything is now out in the open.  Tell the old drummer what is acceptable and not.  Make a date to followup with this issue.  The old drummer will either quit on the spot, agree to be more responsible, or do nothing to change.  You will then have your answer.  
In business, the HR department is obligated to follow these kind of steps with the current employee who is falling short on their job responsibilities.  You always start with communicating the issue, followup, followup, and then decide if it can work in the long haul or not.  At least you give the employee a clear understanding (verbal and written) on the issues.  
Good luck.

Thank you Rimshot.  That would all be good, if the issues were strictly behavioral - but most of my reasons are for ability.  That is just something that, after 30 years of playing - if you haven't acheived it - you never will.

bapu
If you decide to make the change send him this:

I'd rather be drop kicked without warning and have my short-ones set on fire.

This thread reminds me of why I decided to stop playing in live bands.
 
With no disrespect to Scott, there is just too much potential for drama (for me) in the live "BAND" situation. I've been there and done that. I was never fired but I have been on the firing side. I did quit one band. The guitarist went on to become a respected producer (now in Nashville).
 
I do have to agree with the notion that a public forum is probably not the best place to discuss personal matters like this.

bapu
I do have to agree with the notion that a public forum is probably not the best place to discuss personal matters like this.

That reminds me, there's something we've been meaning to talk to you about...
 
I guess we should start a new thread for it.  

Bapu - you are COLD! 
 

http://www.reverbnation.c...-sleep-deprived-sample

Here is a sample of this drummer in action. It is hard to believe he is only 20.

Nope, blank page Scott.

Fixed it!!

This probably won't be to much help in your exact situation but you may get an inspiration or two from this link about the Beatles drummer issues. Good luck... it always hurts to hurt a friend.....but sometimes it just has to be done!!
http://blogcritics.org/the-beatles-many-drummers/