Has he admitted the offence?
The reality is this I'm afraid.
Firstly they probably would contest whether he needed an appropriate adult anyway.
Secondly even if you got over that hurdle the only affect it would have would be to prevent them relying on the interview. It wouldn't stop them bringing a prosecution and they could rely on the findings of the blood sample as they wouldn't have needed an appropriate adult for that.
I'm very sorry but that is the reality of this situation. I'm afraid that it is quite hard to get these things excluded now.
Can I clarify anything for you?