If the OP hasn't gone away, here is some information from someone who has been on the school side of the table in a number of these sort of meetings (in a Governor's Disciplinary Panel).
First, check the school's drug policy. It will be (or should be) on the website. That will detail the rights of anyone that contravenes it.
Second, despite a school having a zero-tolerance policy towards drugs, unless a student is dealing or using class-As in the dining hall the majority of schools will arrange a number of meetings - with the local community police, with a drug counsellor and with the Governors to help the student understand consequences, and to know where to get help.
You say this is a resolution meeting - this sounds like part of the process. DO NOT go in talking about rights until you have cjhecked the policy and know exactly what it says, as a Governor I can tell you the school will be putting the rights of the other students to be in a drug-free environment first. I would expect the student that supplied the joint tobe excluded - or put on notice of exclusion. Your DS should just get a warning.
I know the Scottish system is different, but the principles will be the same.
Oh, and by the way - I don't believe a toke on a joint is a hanging offence, but all rules are there to be followed, and those that don't need to accept the consequences which are (or should be) clearly laid out and trump the 'rights' you quote.