Just Sayin

It’s not rude to ask imho as long as you ask nicely.
I had the same concern about a month ago. I asked a masseur (a regular) if he had any availability on X day. He said he was pretty booked that day but asked me what time was I thinking that I wanted to come in. I said I could come in  between X and Y hours but that “I’d really prefer to see you when you’re not so busy.” He said unfortunately it’s often like that unless you book in advance. He then proceeds to offer me a few optional dates on which I would get his “full energy.”
The masseur knows his schedule best and if you are respectful he may try to accommodate you. No guarantees but worth the try. 
 

I avoid booking appointments back to back. It’s too stressful and I need a bit of time to wind down and prepare for the next client! Back to back sessions draining! 
 

Not at all.  He may actually like it because it allows him to see others that don't have the same concern.  Depending on what you're looking to do he may prefer to see you first or last or on a day of the week where he isn't as busy.  Could work out well for both of you.  Just talk to him.