EEA family coming to UK with one child studying in Holland

[JM98]

Dear All,

I am Dutch and grew up in the UK from the age of 12 and left the UK aged 27.
(financially independent)
My Mother lives in the UK (since 1979).
I am planning to move back to the UK with my wife (Luxemburgisch) within the next 12-18 months and understand that the interim agreement would allow us to do this without issues (should a deal be made) or at least without issue if before the end of March 2019.

One of my sons (age 19) studies in the UK (since 2017) and is planning to remain in the UK.
Will his study years count towards the time required to qualify for Indefinite Leave to Remain?

My second son studies in the Netherlands. (he is 21). Will he be able to join us after his studies. (His ordinary residence is with us (ie will change to UK) but he spends most of the time in NL because of his studies.)

My goal would be to become permanent citizens and citizens.
I hope some one can give me some guidance,
Many thanks in advance,
Jaap