Your son and my daughter(8) sound very similar. She has similar tastes in books and similar sensitivities.
She loved the Owls of Gahoule series(sorry the spelling may be off), currently she is reading the Warriors series by Erin Hunter - I would have thought these would have been too much for her, but she doesn't seem to be bothered by them. She also loves the Bobbsey Twins and Nancy Drew books. We don't have any Hardy boys books here at home, so I don't know if she would like these too, but I suspect she would.
I can't keep up with her reading any more, so DH or I skim random parts of the books she picks and see if we think they are appropriate. She has gotten really good about stopping a book if it is getting too much for her.
She is desperate to read the rest of the Harry Potter series, but like you we haven't let her read past book 3.
She read Percy Jackson and the Lightning Thief, she liked it, but hasn't gotten any more of them from the library, so I'm not sure what was up with that series.
She also read a book called the Red Pyramid in school, and loved it. She wants to get it and can't wait for the next one in the series to come out.
Have fun searching - it can get pretty tricky. I have found a few librarians who have been great finding books for her, so you could ask around too.
