Hi, OP -
I am a STEM academic. DF is an engineer, now retired, and DH is a maths professor.
You have loads of good advice above and I agree with most of the actionable aspects. In particular @TizerorFizz is a goldmine of useful information on all things related to CE.
I was moved to write because this is a fascinating dichotomy, and somewhat because (being strongly maths adjacent myself) I want to object gently to the idea that one field or the other is more creative. Maths is wild and exciting, albeit not maths of the school (or perhaps the UG variety, though some of this can get very interesting IMO).
We need more engineers, so good on DS for having this as his career goal. But how deep is his love for maths? If he would strongly prefer to study maths would he be more likely to get a better result with it? This is important. He could orientate his study towards engineering problems if desired. The border between applied maths and engineering is permeable.
Maths graduates get excellent jobs. A few years ago MIT surveyed their new graduates and Maths came out tops in earnings with Computer Science second. (Both around $120,000 pa IIRC, but this was MIT). However there is no obvious career path like engineers or computer scientists have.
I am not trying to persuade DS out of a career in CE. But if there is something else he would prefer to study, and that something can link up to CE, it seems to me worth exploring, and now is the best time to do so. (We need mathematicians, also.)
DS really cannot go wrong between the two fields, unless he ends up studying something that does not suit him.