How common is it for a professor to agree to review a bachelor thesis outside of his subfield? I am asking for a professor to review my undergraduate thesis. I like him a lot and I don't want someone else who is an expert on the field.

How common is for a professor to accept to review my Bachelor thesis outside of his field, in computer science?

Is there anything I can do to maximize my chances of him accepting?
I don't think it really matters if it is common or not. If the professor feels comfortable doing it then he will do it. If not, than he will tell you. 

As Bryan said, it depends on what you are defining as outside his "field". If he is an art professor and your thesis is in math, than he probably won't do it. 

I was in a similar situation to you. My undergraduate thesis was about how to choose landing sites for rovers on Mars, but my professor had never done anything like that, however he was a geography professor who used satellite imagery (something that I would use a lot of as well) so there was at least some overlap.

I'd just go and talk to the professor. If you like him, and feel he's a good guy than I'm sure he'll give you an honest answer and won't hold anything against you for asking.