A year ago I did an REU with a professor at a school different than the one I have attended as an undergraduate. He has written me letters of rec in the past (which seem to be strong based on where I’ve been accepted), but I am wondering what the etiquette is in asking for letters for grad school. He suggested that I should come back to that school for grad school, but it is not my top choice, mostly because there was not much mentor presence while there. I would like to apply to a number of other schools, but also apply to his school (his would probably be about 4th on my list). Would it be a conflict of interest for the professor if I ask him to write me a letter for the other schools and for his own school? If I would apply to his school, it would be very obvious that I would be applying to his lab specifically, so I feel like it would be awkward to ask for letters to other places.
In your email, say that you are planning on applying to X, Y, and Z schools in addition to his university and ask if he would be willing to write a letter. You could also ask if he had any additional suggestions. It is normal to want to try someplace new.
I would see no conflict of interest at all in this situation. Professors know that deciding where to go to graduate school is a big decision with many different opportunities, and I would venture to say that the professor would tell you to explore as many options as you need in order to go to the place that best suits your interests. After all, he was in your shoes at one point too.
I also agree with the above post that says you should be up front with him about the places you are applying. He may have some interesting insights about those programs that he might share with you which is only more information that can help with your final decision.
My undergrad PI wrote letters for all my PhD applications - including my application to my undergrad institution, knowing that staying to work with her was sort of my back up plan. She completely understood my desire to branch out and go somewhere new (though I’ve actually ended up in the same city) and wrote me glowing letters of recommendation. Your PI should have no problem writing letters for other schools.