How to amicably bow out of a responsibility where there was no payment of services or contract?

I returned a favor to an acquaintance by helping him build a website. He now refers to me as his "guy" for the project of establishing some other functionality to it. I slowly agreed to more help, but after dealing with him I no longer want the burden. It was all pro bono work, I just find the dude is very unpredictable to work with, does not follow his own scheduling and frankly his further expectations are that of a salaried employee. I had no problem doing the first half of the website and I believe it turned out well, but I have not seen it as I've been ducking his calls.

He has been calling and I have been a pussy to answer. He will ask me to pursue more work that I simply do not want to nor have the time to do for him. I don't want to ask for pay to do it either, I just want out. I had agreed to see his website through the summer.

I feel indebted to him, which makes it strange, and he is a nice enough guy. I don't want to sabotage the friendship. He just called me 3 times and its getting weird. His messages are typically "hey call back asap" or something similar that a boss would say to his underling. He calls during business hours, several times in a row expecting you to drop everything. Then he won't follow up when he says he will. How do I exit this situation amicably rather than just dodging him like a bitch?