I notice you're spending quite some time here, and more than one post, needing guidance for your son.
May I make a left-field suggestion instead: log into as many job search websites as you can find, and search for LOCOMOTIVE DESIGN ENGINEER job applicants, job openings, job offers and any 'mentoring' offers. Read carefully what these hopeful employees are offering as skills, what these employers are looking for in skills for the jobs offered, and seek out any opportunities your son can join and explore more on his own.
Unless you two are only looking for a drafting/drawing non-professional career path in which case I recommend a visit and some classes at a local community college after visiting with a counselor for guidance.
AutoCAD has no future in that industry. Inventor is not something you can just pick up without hands-on training and guidance. Other software packages not offered by autodesk may be even better for that industry, I doubt anyone around here can easily pinpoint which.
Good luck; you sound like a great parent.