Son should have followed later but was refused visa (apparently he could have lived with his aged and infirm grandparents

Wife’s FLR was due soon after son’s arrival and both were granted FLR.
Wife’s ILR is due in September 2021 but son will be 19 by then (in full time education and living at home).
Will son qualify for ILR by being “attached” to his mothers visa despite living in the UK for only 16 months? His BRP expiry date is Jan 2022 same as his mother’s.
If yes, I understand that he will have to apply for ILR independently from his mother as he is over 18. Would that be application SET M, SET O or some other?
I assume he will need to be sponsored as a dependent by me as he has no income.
So, how will the financial (income/savings) details be calculated/applied? Can the same details be applied to mother’s and son’s application? (ie £18,600 income for each, or submit cash savings of £62,500 on both?). Or do you need to show income of £22,400 for each (£72,000 cash savings)?...... or something else?