
Does anyone know what his tattoo on the back means?

I'm gonna preface by saying I'm no expert and I'm very much guessing with this bc the first character is particularly difficult to make out (at least to me), but I'm guessing the characters are "新海" (Shinkai in Jap, or XinHai in Chinese). You can do your own Google search if that makes any sense, but individually the characters translate roughly to "new" and "ocean".
I would ask someone who is more familiar with reading characters to look more closely especially at that 1st character though bc I have low confidence I picked the correct one... But I tried 乁| ・ ・ |ㄏ
Wooohooo another misunderstanding arc ahead