Explanation:

Mad about vs. Mad at:
When we say someone is "mad about" something, it means they are passionate or enthusiastic about it. In this context, the son is very interested in science and technology, which is why the correct preposition to use is "about" instead of "at."

Choice of career:
The sentence suggests that the son's interest in science and technology is so strong that he may choose to become an engineer in the future. This shows that his passion for these subjects is likely to influence his career choice.

Engineering as a career:
Engineering is a field that requires a strong foundation in science and technology. By expressing his interest in these areas, the son is showing signs of a potential future career in engineering.

Parental observation:
The speaker, who is likely the parent of the son, is making an observation about the son's interests and future career prospects. This indicates the parent's awareness and support for their child's passions and ambitions.
Overall, the sentence conveys the son's strong interest in science and technology, which may lead him to pursue a career in engineering. The correct preposition to use in this context is "about," as it signifies enthusiasm and passion for a particular subject.