The repetitions would indicate that he is not using a shifting key.
I will bet you dollars to doughnuts he is using letter/number replacement - possibly rotating but if so only one-wheel and/or with another simple substitution set - on a standard prison “tap” code, i.e. break the alphabet into a grid or reverse grid, etc. and make “F” for instance 23 (or BC)(2 across three down on the grid.)
I cannot imagine it would be hard to crack any code that one man keeps in his mind.
A high-school teacher and his wife broke the Zodiac code with 20 hours of effort.