Promised? Recruiters can't promise anything.
When I first took the test at the recruiting station, I was told that I scored high enough to be eligible for any rating. (The recruiter said it was one of the highest scores he had seen but that was probably malarkey.) Then, during boot camp we took another test before we filled out our requests for school. Again, my scores made me eligible for all ratings (but I could only apply for ratings that were open to women).
So, I applied for journalist, photographer's mate, and aerographer's mate, in that order. I was told that I probably wouldn't get journalist because it was such a closed rating that hardly anyone ever got it, especially women. Surprise! I got it!
Bottom line, it's whatever the needs of the service are at that time.
Nuclear engineer is considered an el primo assignment. It's not easy but it's prestigious. The school is very long, and the payback time is also long.
So, what did your brother do? Did he accept the school?