Depends really what catches his fancy.
Ds is 13yo.
He's just spent several late reading nights finishing the Henderson Boy series by Robert Muchamore. He'd finished the Cherub books (Henderson Boys are a kind of prequel) first.
But it depends on your 13yo. There's a lot of violence, drugs and a rather poor attitude towards girls by the main character at times.
Alex Rider (there's a Netflix series if you want to whet his appetite with that) is a similar premise but less of the above. I think a good way to put it would be in these books you'd have "Alex swore", but in the Cherub you get the actual words at times.
Alternatively you might find that something close to HP is the Chrestomanci series by Diane Wynne Jones. They're quite fun.
The Narnia series has the same good v evil fantasy, ditto the Dark is Rising series.
Ds has also enjoyed older books, like Biggles, 3 Investigator and that sort of thing.
If he likes fiction written about fact, then Josephine Tey's Daughter of Time is a modern day investigation about the Princes in the Tower. Ds loved it, and I pity any history teacher trying to teach him about it!
Ds has enjoyed all these.