In response to your first question, Finance and Econ tend to be the two most common majors for IB incoming interns so doing an Econ major on a standalone basis would certainly keep the doors open for IB recruiting. I double majored in both finance and econ and found that the coursework from each major was helpful for preparing me for a career in IB. However, would note it does vary by school in terms of what major best prepares someone for IB recruiting. For instance if at an Ivy or top LAC, there likely isn’t a finance major available so econ is what most people who are planning to recruit for IB tend to major in and many of my coworkers were econ majors at top LACs. However, at a school that offers an undergraduate business program, finance majors from the business school will typically have the best placement into IB (in part due to the finance curriculum but also due to the stronger emphasis on career development and finance extracurriculars in undergraduate business schools, plus alumni and HR will typically give the edge to the business school students) compared to econ majors in the school of arts & sciences. However, if the econ major contemplated is housed within the business school, then this distinction is not really as significant.
For your second question, participating in finance-related extracurriculars whether through your son’s university or externally is definitely important and will help make his resume more competitive as it demonstrates an interest in finance and perhaps will enable him to participate in some professional experiences or deliverables such as stock reports or case studies. When we are going through resumes, we definitely see a lot of candidates with finance extracurriculars so I would say that it doesn’t necessary give an edge in recruiting but more just keeps you competitive amongst the other strong resumes. Examples of the types of finance extracurriculars students have on their resumes: IB workshops/bootcamps through their university, student-run investment portfolios, M&A clubs, distressed investing clubs, as well as third-party programs such as SEO (diversity) or corporate sponsored externship programs, case competitions, virtual development events, etc.