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Testicular tumors have a similar behavior to those in people. They can grow quite large and have endocrine secretion to lead to other signs. The chance of metastasis to other organs is pretty low, though. It's thought to be < 15% as far as chance of metastasis. With that in mind, surgical removal is often times curative for these tumors.
I would not hesitate to neuter him and do a scrotal ablation. This is where the scrotum is removed as well. This gives you your best chance of removing all cancerous cells. The surgery should be curative in 85% of dogs.
I hope this helps, let me know if you have any other questions.