I would ask myself what this angel/investor put on the table except for money. If it's just money, you can use some evaluation techniques to calculate how much he deserves. If you think he will promote to the company other than its money, I would build an equity structure that will make it happen. I have some concepts in mind, we can further discuss it.

If this employee is important for you, my suggestion would be to discuss it with him, to share with him your thoughts and to agree together on the stake. It is so valuable to put the employee, especially in early stage, in the loop. It will make him more involved and he will appreciate you. Of course, come to that meeting with a range of % you are willing to give.
We can continue our discussion and share with you my last experience which was the same.

In addition, you can google for websites such as cofounderlab - https://cofounderslab.com and more