Here's the basics on equity:
If he writes a check for 50,000, he will write it from him TO the company. The company will then have 50,000 in the bank. In exchange he will get a certain percentage of the company. That exact number depends on the negotiations between you and him.
Premoney valuation: This is the value of the company before he puts money into it.
Post-money valuation: This is the value of the company after he puts money into it. It will always be premoney valuation + amount of money.
His percentage will depend on the premoney valuation.
If your company is worth 100,000 before his money, then his 50,000 would make it 150,000 total. That means that he has 50,000 / 150,000 or 1/3 of the company. You would retain 2/3.
In the future, if the company sells, he will get x% of the sale price (1/3 in the example above).
If you make a lot of money and decide to withdraw some from the company, for every 2 euros you take out, he will take out 1. (if the numbers above hold).
For 'idea' companies, coming up with a pre-money valuation is extremely difficult and purely subjective. You will need to negotiate this term.
Once he owns a part of the company, you will likely never get it back. Only allow him to invest if you NEED the money to get started. If you can, fund the company yourself and keep 100% of it.