Ok so seeing that your engineers you should find his favorite gizmo unscrew it to open it or whatever and grab a hammer and just start laying into the inside workings of it. I mean really make sure it won’t work upon starting. And stash a love note inside and close it up so the outside looks perfect. Then when he realizes it’s not working (and with him being an engineer) he’ll naturally want to inspect the insides to fix it and find your note. Hope this helps
Throw hundreds of bricks at the AC system with love notes written to him by name. Anonymous would be best because then you can capitalize off the swinging emotions going off in his head about who would do it. Now you can tell him that it was you if you think he can keep a secret because y’all sharing a secret builds intimacy.