I don't know how to ask him out.

So, I have this guy friend who I like and he likes me. I want to ask him out but I am scared to because of the age difference. I'm 19, almost 20, and he is 34. My parents don't know about him. We have been seeing each other in private. I don't know why I'm scared and feel like he will reject me. I feel ugly and stupid. What do I do?