Similar with my DS, also 7. Used to not mind either way but now has become resistant to 'girl books'. Sigh.
Before this happened, he liked e.g. 'The Secret Rescuers' series. Lots of adventure and mythical beast, just like in his beloved Beast Quest... what does it matter that the hero is actually a heroine? And is rescuing rather than defeating the beasts? Just a shame that the books are pink and sparkly.
He really enjoyed 'Little House on the Prairie'. But may be too wordy for yours. You could read it TO him though?
Very short and simple, but the Magic Treehouse series has a boy/girl combo; each has their strengths and specific characters which are not too gendered in an obvious way. The boy, though older, is cautious and wary; the girl is brave and outgoing.
I'm going to try DS on 'Ronja the Robber's daughter' one of these days. As there is a recent movie out, which looks fun; I'll apply our usual rule that he may only see the movie after having read the book, as an incentive.
I find 'classic' children's literature often has boy AND girl characters, and it is the group of them that are all equal 'lead' characters. E.g. Famous Five, Secret Seven, Magic Faraway Tree/Enchanted Wood; Five Children and It, Swallows and Amazons, ... However within their group, they have very stereotypical roles e.g. always the girls doing the cooking and mending and tidying, always the boys bearing responsibility and making major decisions.
In more modern books however I find that though there is perhaps less stereotyping of the characters, there is more stereotyping of the reader IYSWIM? Often there is either a male lead, possibly with a female sidekick (or a mixture of several sidekicks); or a female lead, possibly with a male sidekick. The books are a lot more marketed towards either boys OR girls, rather than at 'children', which can be seen by the covers, at a glance.
Not sure which I dislike more!