What's nice is that even with the lot you've already gotten, he can do quite a bit. I would recommend either a gift certificate to a local or online electronics hobby store, or depending upon the retailers in your area, you might have a GOOD electronics store with employees with at least a passing idea about what they sell.
If you do, then you may be able to ask them in simple terms-- boyfriend new to electronics, working with a microcontroller, and you'd like to get him an inexpensive basic component assortment. Typically that would be a resistor assortment in the most commonly used types, as well as capacitors, a few transistors.. LED's, etc.. basically expanding upon the kit you already purchased. What you are looking for may be sold as "experimenter's kit" or "hobbyist kit", or possibly "student kit". It should NOT be a ton of money, I've seen setups like this in the $20 range, that were pretty well set up for the price. The best deals on this sort of thing are online, but local shops sometimes still remember experimenters exist..
Beyond that, I think it's probably best to see where the little beastie takes him.. some get into servos and motors, some web-based stuff, some into just about anything. He'll be dumpster diving in a week anyway, and will have Arduino-ized both the toaster and the cat by month's end...