Read about your predicament and I shall share some of my thoughts from my personal experiences.
1. You said your DS was diagnosed with ADD and then managed to be promoted to Express in sec school. This I find is an achievement as really not easy.
My grand niece now is in P5 (next year PSLE). From my observation, she has some degree of ADD (‘slow’, poor memory retention, behaving like almost a mute meaning don’t ask questions at all, etc etc). No formal diagnosis coz she is not ‘disruptive’ type.
I give her ‘specialised tuition’ as I assessed outside tutors can’t help her much. She needs ‘filtered materials’ and not all materials with my aim of getting her to score well enough to be eligible for NA promotion. I only tuition her about 4-5 hours per week for 3 subjects (once a week only when she comes my house from 12 pm to 8 pm on weekend when I will plan activities for her).
So far so good that both of us happy that she is hitting her NA target (genuinely having very good and trusting relationship among her and my family members). She will lose careless marks here and there but those are expected and so I don’t get into nags or ‘despair’.
She is strong in sports and so we have a current consensus that she will have to clear her O levels to be good enough for a sports management diploma (and I am very confident that she can make ‘good enough money’ in a job of her interest probably as a sports/yoga/pilates/etc coach).
2. You said your DS passed all Sec 1 subjects and again this to me is no mean feat. My nephew then failed 4 or 5 subjects in his Sec 1 final exams (he still got promoted to Sec 2 Express). This nephew is highly intelligent but fell into arcade gaming with friends at that time. Even then, I didn’t engage any tutor for him as I assessed it was an attitude issue with nothing wrong with his intelligence. I had a good talk with him (he doesn’t have a mum) and told him we would have to ‘experiment’ with tutors by the end of Sec 2 if his results didn’t show improvements.
He hates tuition and finally made an effort to study on own to avoid tuition (he had to learn to juggle among his studies, play, and sports). His L1R5 is a single digit for O level and now a very sensible 18 yo boy who ‘respects’ me a lot (will bake cookies and spend 45 minutes travelling to my house to give me kind). My patience with him reaps boundless rewards.
Do not despair and do not compare (with those ‘normal’ kids) basically. I seeing your DS as ‘good enough’ and I hope you can see him too from a better light. Your DS is really good enough and deserve a big hug! (your kinder words and attitude may make or break him as in whether can grow up to be an emotionally stable person or not, and so this would be an area that I would focus more over academic results, coz with emotional stability, other ‘achievements’ will come naturally).
(BTW, don’t need to pass ALL subjects. If can’t cope, by end of Sec 3 can start to ‘filter’ out weakest non-relevant subjects to prepare for O levels. It can be easier if you can let it be.).