Careful getting into defence industry, this could be your son later on life
Agree mechanical would give a broader appeal to employers but electrical/HV mechanical skills/experience would give a more broader appeal while also future proofing him until retirement
My son is currently doing his internship, working at a small motorsports damper company, he is absolutely loving it, has already worked on GMA T50 hyper car, Dakar T1, Formula E, Singer DLS, USA Subaru rally cars. He chose it over an offer of working on restoring, building classic Group B rally cars
No internships were advertised, he just contacted companies, knocked on doors and created his own opportunities
Best bit of advice I can give your son, is to target the field of engineering that interests him the most, and just go for it!
Do the research into which companies, do what, mine LinkedIn for names/message people, email resumes out, phone, knock on doors, ask friends/family for contacts
Keep repeating, ignore any no thank you's, no replies, their loss, not his!
Good luck to your son and please keep us updated