@Hejti Well, I would like to point out that in chapter 17 we never see his face. Granted, we also don't know if Gen is the same age as Reiji, but assuming he isn't it could be that Gen was like 5-6 and Reiji is like 4-5 at that point. I was wrong earlier about the part of Rei being younger.
Interesting she has an eyepatch, but her eyes seem fine now. Also there's some circumstantial evidence to connect the two such as the aforementioned fact the author left the town and knows his mother, the fact both Rei's father and the author are smokers, the fact that we know that the author is abusive and has been divorced multiple times, they both have the same hair color, the fact that Reiji is said to remind people of the author, etc.
Also I think it's confirmed that Reiji doesn't know much about his father given the fact he didn't know that Gen's dad beat up his dad, and that he didn't know his mom's relationship to Gen's dad, which indicates to me he doesn't remember his dad at all.