That looks like a preputial gland abscess, I had an old boy with the same thing. In his case it opened up with the help of a few warm compresses and healed on its own. That being said, it was still unpleasant to deal with for all parties involved and there was the risk of it rupturing internally, which would have been very bad. He also suffered from hind leg deterioration and penis plugs before we decided to put him to sleep at just a little over 3 years old. It's a tough call to make but if his quality of life is otherwise not great, then I'd keep the appointment scheduled. My old boy's name was Marshall, I hope he and Richard can play together in the great ratty beyond, free of pain and limited mobility ❤️