I'm studying in engineering and so is my boyfriend, my brother and all of my friends. They all love Skyrim (it's a computer game) but your boyfriend is likely to have it already since it's very popular. Minecraft is popular too. And they all love old school gaming consoles (Nintendo 64 especially, but it might be hard to find). I don't gift my boyfriend some "engineering" stuff though. I think about what he likes the most instead, like photography stuff or stuff for camping and outdoor activities. Just like this other guy said, focus on what you know your boyfriend likes. Of course if he's in engineering he'd find a use for a motor or a beiring - something like that - but it doesn't mean he'd like it more than anything else.
Also, last minute idea, a flying simulator. All aerospace engineers crave that thing. I think it's around a 100 bucks or something. It comes with a special headpiece and a joystick and it allows you to control the simulated plane as if you were flying in real life. When you turn your head, the camera turns with you on the computer, etc. If he's into that, I'm sure he'd absolutely love it.