Can you explain your situation a little more?
I'm afraid they don't have to tell you anything at all.
He is an adult and without his permission you cannot be given that information. The police are bound by confidentiality at least at this stage. Sometimes they will disclose the name of the solicitor's firm but nobody can force them to do so.
The solicitor will speak to you only if he has the permission of your son as he is bound by professional confidentiality.
Ultimately though, it cannot be demanded. Unless the police will be willing to disclose that information there is no way of forcing them to do so.
I understand you are worried but your son's a right to have his confidence respected.
Can I clarify anything for you?
Is there relevance in the time of arrest? Are they saying he responded in some unhelpful way?
I'm happy to continue with this but please rate my answer.