IMO you definitely have a right to be there if your son requests your presence and witness.
There is no reason why you cannot claim to be his legal advisor, Nor is there any reason your son cannot appoint you his legal advisor, if you cannot afford someone from this web site.
If the police make excuses why you cant or should not do something your first question is always "what law says that? Show me that law." Anything the say accept accurately answer your question is rubbish. Write down whatever they say to you and tell you.
What law exempts the police from obeying the law ????