I have 2 adult sons with dyspraxia and ASD; one also has ADHD & dyslexia. Both very clever but struggled to write. I also have a 12 yo DD with ADHD, dyspraxia and dyslexia.
School doesn't get easier; in fact it gets much harder, and yes the gap does widen. But kids with SEN are entitled to extra time in exams, which should help. DS1 got really good GCSE & A level grades but dropped out of uni during the first term because he couldn't cope with it. After a few years working in retail/fast food he applied for an apprenticeship in Engineering and has flown since. He bought himself a house and lives on his own; something we never thought he would be able to do.
DS2 also did well in his exams and went to uni. He got lots of help from the Disabled Student unit and really enjoyed his 3 years, but failed the final exams so didn't graduate. He has had loads of jobs since, got married and had a child.
You really can't prepare for the future because there is no way of telling what will happen, and you could be worrying about nothing. Your best bet is to concentrate on the now, and supporting your DS in what he is doing. Ask for a referral for an ADHD assessment; meds can make all the difference. Speak to the SENCO to see how they/you can help him.
Find him some out of school activities to help boost his confidence and burn off some energy. My DD had 1:1 swimming lessons when she was younger. I also found that music groups worked well for her, the sort of free-flow unstructured arty kind, rather than traditional music lessons. You could try martial arts, yoga, climbing, golf, gymnastics, trampolining, dance, singing; whatever his interests are.