If he has an MSc or MEng and is 30 with a reasonable number of year experience (5+ post graduation is sufficient), he should be a Chartered Engineer in the UK, or local equivalent (I've no idea about Taiwan but this would be a Professional Engineers' Licence in the USA, Eur.Ing. in a lot of the EU, and so-on). That puts him in the position of taking reasonable management responsibility, sign off design decisions, and so-on.
I would suggest that for international portability he gets full earned membership of one of the two major international bodies: either the American Institute of Aeronautics and Astronautics, or the Royal Aeronautical Society, and the associated "charter" either in the USA or UK, or his home country - or both. (If he doesn't mind the paperwork, all three). He can get Eur.Ing. from the UK if he has CEng and a second European language, just on paperwork.
After that, really what matters is not further qualifications beyond that - it's skill and experience. The simulation industry is global, and if he has the skills, getting permission to work in a foreign country is essentially a matter of being offered the job because he's the best man for it, then paperwork.
He absolutely should be engaging with the profession - both RAeS and AIAA have simulation specialist groups he can join, and organise conferences he can attend. This is how you get known and build a network you can trade on to move into better jobs in that sort of community.
G