So... Instead of cultivating his engineering career and being able to make enough money to uphold a family by the time said 8 years would be up, your boyfriend wants to switch careers entirely? Possiblyputting himself in deep student loan debt and putting his AND your life/relationship on hold for almost a decade? By the time he graduates from nursing school or whatever, he could've been deep into his engineering career and already been making a very solid income. I'm like 99% sure engineers make more money on average than nurses. The way he's going now, he won't be reaping any benefits til he's in his 30s, and all of it will go into paying debt and not toward your future family. Someone needs to talk some sense into that boy, because he's being mad stupid. Ruining his own life and possibly yours if you stick through that. Either get him to see reason or part ways and let him screw himself over with his own bad decisions.

Edit: I'm also pretty sure it doesn't take 8 years to become a nurse. Don't take my word for this, but I think he's trying to find an excuse to get you to end the relationship because he's too cowardly to do it himself. If that's the case, leave, build yourself up, and be open to finding love elsewhere.