Welcome to you and Dexter! We're glad to see you on YT.
In my opinion, mucus and discolored stools are reasons to go to the vet RIGHT AWAY. That isn't normal, and the smelly gas is probably causing his tummy to hurt and make him uncomfortable.
Is there another vet you could call and get him in to see today?
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He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion. -- Author Unknown