Child ILR application SET(O) or SET(F)

[ideoma]

The situation

1. I and my spouse both have ILR after 5 years on Tier 1 (G) since a year ago.
2. We have a son as Tier 1 (G) dependent born outside of the UK. He could apply with us but I believed that he could skip ILR step (seems to not be the case).
3. I and my wife have both applied for naturalisation but need ILR for the son now.

The question is what root to apply for the son's ILR, should it be SET(O) or SET(F)?

If I can use SET(O) is section 5 (income) relevant and proof of cohabitation needed? I obviously don't have any bills in 7 y.o. boy name.

If it's SET(F) than more support documents needed, like birth certificate and proof of my permanent settlement status and the later poses a problem. The ILR approval letter is very vague and does not tell that application approved really only that my documents will be retuned. Posting ILR card is also a problem since the naturalisation application can be approved first and I wouldn't be able to return the BRP and face a fine. Will I be able to have an exception if the ILR card hold in Home Office application?

Thank you.

P.S. If anyone can message me HO enquiry email it would be very appreciated. I seems to be a secret of Polichinelle.