I understand where you are coming from. We all want our children to benefit from our experience and fly but life is about the journey and not the destination.
I also have adhd. I wasn't diagnosed until my 40's. In school I was always considered bright and but labelled as lacking ambition. I was ambitious but generally bored senseless by most of my subjects. My reports are littered with comments about how if I would only apply myself, etc. quite a few of my teachers actively disliked me because of this. I can understand why now.
The subjects I did well in (English, History) I was obsessed with. I would read widely outside the syllabus and study independently on topics that were not even offered. The rest of the subjects were like dragging my body over broken glass. Nothing would motivate me to get stuck into them.
Unsurprisingly I didn't get the grades needed to do the course my family wanted me to do (law) and rather than pick another subject I went to work at the local council and stayed there for 5 years. It turned out to be a job I loved (in the libraries with ALL the books!!). It was good for me and I grew up a lot in those 5 years.
At 22 I was ready and reapplied to uni as a mature student and did a degree in a subject I actually loved (archeology). It was so hard, keeping up with everything and working at an insurance company call centre at night as well, but I managed it and did really well.
After I graduated I was planning to go on to a post grad degree but was so burnt out by the undergraduate years,
I postponed it for a year and went travelling. When I landed in London I was offered a temp job at an insurer in the City.
I ended up staying in London and now, 25 years later I run the claims team for an insurer, dealing with high value, litigated claims internationally. I am very well paid, professionally recognised and respected and I absolutely love my job. It plays to all of my strengths.
My career and life trajectory has not been a straight line. Nor has it been easy. My undiagnosed ADHD caused significant difficulties and hurdles in work and my personal life which I internalised as moral failings on my part for a long time but in the end I learned to fly, in my own time and in my own way.
None of my choices were ones my family would have made and I know they were devastated when I didn't follow the path they had planned for me but ultimately it was my life and I had to make the choices that were right for me at the time.
The point of all of this is to say that he needs to follow his own path and ultimately he will define his own success. You cannot force a square peg into a round hole or right the wrongs of your own life through his.
If it helps at all, going into the railways may not be the dead end you think it is. If he is bright he will rise. It is an industry that desperately needs fresh faces and new blood so it could be a great opportunity for him to be in the right place at the right time.
If he could find a way to make my commute less of a nightmare then that would be awesome!