My bad (in part) skimmed your earlier post and did not register that there are now 2 separate returned NIPs in play. That said, most posters would have ensured that this was more prominent.
Many forces will happily offer those who the suspect of concocting a "wizard wheeze" a chance to correct their "oversight". We now have two apparent attempts to pervert the course of justice that have come to the police's attention, that we know of so far. The Met aren't always regarded as the sharpest tools in the box, but you are quite likely to be very much on their radar now.
I appreciate that you don't want your son to tell you anything that you might have to give evidence against him about, but you need to have a word with him to confirm that when you nominate him he's not going to have any more bright ideas, and that the same applies to any other subsequent NIPs. Alternatively, ensure that he knows how to hold a bar of soap without dropping it.
It might be helpful for both of you to understand that whilst an adverse inference can be taken from a refusal to answer questions, or failure to mention something that you later rely on in defence, no inference can be taken from a refusal to answer questions, etc. before you have had chance to obtain legal advice.
