Tabby McTat is great in that it includes a lesbian couple but it is too young for the OP's DS. It is fascinating how picture book authors seem less constrained in what they can do than authors in the middle year age range. There is a book by Hannah Moskowitz called Marco Impossible that looks like it might be suitable though as it is aimed at American middle school (9-13) kids and she is a good writer. But otherwise all LGBT-themed books seem to be for 13-17 year olds and most of those are American. It passes understand why more children's writers don't include gay characters in their books.
There's The Case of the Stolen Scarab and its sequel The Case of the Vanishing Valuables (can't find that one on Amazon UK) -- sort of Nancy Drew-type mystery series but the protagonists have two mothers.
Also A Clear Spring (protagonist is staying with her aunt and her aunt's partner) or Box Girl (protagonist lives with her father and his partner). Depending on his tastes Box Girl might be slightly old for him, because it's more touchy-feely rather than full of action, though it's not really a teenage book either.