First of all, I agree with the professional help. Accountant; Lawyer; Either way, they are taking a step back to look at the problem.
Second suggestion; Take a page of paper with a line down the center and put "PROS" on one side and "CONS" on the other. Believe it or not, when you write things down, you get a much clearer picture. Use your head for thinking... not for storing! Ok?
Now, you should have a partnership agreement with a clause (typically called a shotgun clause) which provides details about how you can or your partner can proceed when you want out.
Now, I can't speak for yours, but typically the agreement would like this:
I am so upset that I want out, I set a price. I will BUY YOU OUT for that price or I WILL SELL MY SHARE TO YOU for the same price. The reason it's called a shotgun is that the person who starts it HAS NO CONTROL over how it ends. He is willing to buy or be bought. If you have that in your PA, then you simply operate by those rules.
Why does he want 10%?? What's that going to do for HIM according to HIM?? That would be worth knowing.
Also, you didn't mention the type of business, not that it makes a huge difference, but if it's like a sales agency where there are not a lot of assets, you may decide you can go it alone. If there are a lot of assets and equipment, that could be a little different.
I don't know if this helps, but I hope it does. I am following this post if you respond.