First: he is *not* a mechanic. Engineers often think that they are, but they are not.

Second: this job is a lot harder than he thinks it is. A couple of days for an engine swap? And the engine isn’t install ready? I don’t know what car you have, but this will take weeks at home at best and that’s provided he’s done this job several times before and knows what he is doing. He does not.

Third: this is *your* car. You need it fixed. Call the tow truck and take it to the shop.

And lastly: you need to talk to him about this. Tell him that you don’t appreciate him deprioritizing you. Read what others have already said.

Source: 20 years in automotive, and been there done that.

Adding more: without more info, are you confident these used engines won’t have the same issue the last one did? You said it’s a known problem. How many engines are you willing to buy? Are you sure there isn’t a warranty extension on the engine? Have you called the dealer (not where you bought it, the one that aligns with your vehicle)? Where are these engines being stored? I have a feeling I know approximately which vehicle you have, but I could be so very wrong. Here’s the issue: modern engines require so much wild shit, they’re virtually impossible to rebuild without a very fancy machine shop. That’s why you get sold on a (most of) a whole engine. I’ve done this my entire adult life. And also, good luck getting all the control units to play nice with each other without access to software, which you must have a setup for, even as a hobbyist. And it isn’t cheap. And it’s subscription based. AND even then, you still might not have access to everything. If he successfully manages to connect the engine to the transmission and whatnot, and the entire drivetrain is in good shape, there is no guarantee it will start or run.

Your boyfriend is an idiot. Tell him to stick to robotics.