Nelliemoser: I agree with you on principle BUT for a lot of reasons (that could fill Gransnet!) if you take a sample of children who don't enjoy reading, there will be more boys than girls.
So, just as we sometimes treat girls differently to give them a boost in other areas, I think we sometimes need to search a bit more for books that will appeal to boys, and make the effort worthwhile.
I suspect OP didn't begin by thinking that a boy needed different books than a girl, but that in trying to encourage him to read, she realised she needed to approach it differently.
OP - I think you may find it changes as his brain matures.
One of my sons struggled with reading - he could read, and did so for school and work, but he found it tiring and not until age 25 did he find that he could read fluently and therefore 'enjoy' reading for pleasure.
I suggest trying cartoon books and graphic novels such as Asterix. Some children like them because there is less text, others find the page too 'busy' and can't concentrate.