If he is into those, he may be into a microscope, too. We've got a Chemistry set, but the kids (younger than yours) use the plastic test tubes, beakers and vials to mix colors (food coloring) and "do chemistry" at the kitchen sink. Any kit with plastic tongs etc would work. For the microscope, we bought the one at Toys R Us (for 8 yrs +) and it works really well. It's easy enough to focus and does a great job. We have had hours of exploration with that gift. We've made slides of all sorts of things, and our son has added 'premade' slides to his Christmas list, this year ("Creepy Crawlies" and "blood and guts").
We bought a cheap metal detector a couple of Christmases ago. While the kids LOVED it, I find the high pitched beeping sound to be obnoxious. Every once in a while we will take it outside to play...but then it goes right back onto a high shelf in the garage. My only advice on that one is to check out the sound it makes BEFORE you buy it.