I'm going to kick a player out of our d&d group because they can't do math
Why? Is math that important with D&D that you need to kick that player?
I mean, the guy can't count his dice
I know it must be frustrating to deal with your friend.  I think the better way for you to deal with him is just to get him a calculator or something.  Rather than ostracizing him then, you're supporting him!
No, it's better to kick him out. It's his fault in the first place.
The point of playing D&D is to have fun with your friends and you will be actively ruining the fun of your friend if you would kick him out rather than helping him solve something if math is involved. I do wish that you just exert a bit more effort into making him learn better rather than banning him from the game.