want to switch DS's interest from pure fictional to science

You could read one of those books to him together, as PPs have suggested.

But if he's comfort reading HP & PJ, and doesn't necessarily have a natural interest in the "big questions of science" but is more fantasy/myth led then I can't see how you'll instill it by asking him to move onto books with tricky vocabulary and themes that are very complex.

If you've a boy who loves reading for pleasure at 11, please don't put him off by suggesting he needs to "move on" from what he enjoys.

He's a 10 year old boy. He reads for pleasure. He's doing pretty well you know.
And he may have absolutely no interest in science; it's not a given that because you have, he will.
Maybe let him find his own passions

Let him read what he wants to read, and find some good TV programmers to watch together for the sciency stuff. Brian Cox, Mythbusters.....there's loads.

He needs to enjoy it. I do see where you're coming from but I worry your approach will erode any joy and pleasure he has in reading.

I'm a librarian (hello!) and can only say let your child pick what they like to read. Whatever they like to read. As long as they keep reading. Take them to your local library and let them browse. Look at covers, read blurbs. Nothing puts someone off a book quicker than being made to read it.

A bit puzzled why you'd want to - science fiction doesn't necessarily teach any more science than other books. The only thing I'd suggest is encouraging lots of "what if?" questions (what if we colonised other planets, what if we could incubate babies out of the womb) as these are often the germ of food science fiction stories as well as good for the imagination and curiosity.

Just appreciating LadyPeterWimsey's username.

Honestly I'd just encourage him to keep reading lots. I wouldn't worry what it is (as long as age appropriate).
I'm still curious about why you want him to read science fiction though? To some extent genre doesn't mean much - the same story with the same characters and same moral and same quality of writing could be set in a spaceship or a medieval castle or a modern house with only 'window dressing' changes.

I once recommended a great (adults) science fiction series to LadyPeter (hi!), and later she teased me for recommending her a book with a spaceship on the cover. It was very funny.

Sorry, really confused - Why is it you'd like him to stop fiction he's enjoying in favour of science fiction he isn't? I thought reading was all about personal tastes and enjoyment?

I wanted my DC to read for pleasure first of all. When I thought their reading was a bit too narrow and they were resisting my recommendations I chose a great book that they wouldn't otherwise pick up, and read them a chapter aloud in the evenings. It was good having time together, and they often went on to read more widely themselves. It also meant they could become familiar with more challenging books, or books about specific issues that we could talk about together - e.g. racism, the Holocaust.

How old is he? I do see merit in gentle encouragement towards a wide variety of different kinds of books. But depending on their age and character, that can ultimately end up counterproductive. And I've found with my DC that encouraging a big volume of books (lots of library visits) ends up with them 'running out' in preferred genre, or getting sick of it, and moving on to a wider variety.

But why science fiction in particular?!

Why though? Reading is supposed to be fun, surely cajoling him to read something he doesn't want to will strip all the pleasure out of it. I would say encouraging him to read a wide variety of books/genres is fine, but if he doesn't want read a particular genre I wouldn't force it.