In addition to what Krishna Chaitanya and Swati have mentioned I'd like you to ask your son to take the following considerations into account:
This might be likely considering his exemplary academic performance. In this case it might be beneficial for him to pick Electrical Engineering since he will be done with his undergraduate coursework one year earlier. Unless he specifically wants to work in pure or applied mathematics, I don't see an advantage of an MSc in Maths. EE courses also cover most basic engineering maths, some statistics and programming and most higher studies programs in areas like CS, Data Science, Electronics etc. would definitely be open for someone with an EE background.
2: Is his main goal placements?
In this case I think applied maths definitely has the advantage. EE is a large batch (close to 150) and there is a lot more competition for the few companies that are exclusive to EE. Most CS companies accept applications from maths students, definitely much more than those who accept EE students.
Other than this, I think the best thing he can do is speak to seniors and ask them about their experiences. If possible avoid talking to 2nd-3rd year students and talk to 4th year students from both programs. They will have the most up to date, current and relevant information.
Regardless of his choice, I'm sure he will do great.