If you cannot make your point in 20 minutes,

what's the point to take the call?

If I were you, I would write him abiut my expectations in a very concise but polite way, guised as the meeting agenda. During the meeting, I would thank him for the offer to start with, followed by raising my points directly and confidently. I must convince him that the offer of W would benefit to both.

You have to keep positive all the time and make your mind about your bottom line rather to take this meeting for your grievance.

Good luck!