My dog has a strange spot on his skin. Is it a hot spot or serious?

It could be a hot spot (moist dermatitis) or an injury of some sort. It is difficult to determine where on Caesar it is located from the image provided.
You have two options. The first is to have him examined by your veterinarian. The area may need to be shaved to provide a better look at it. Afterwards, your vet can diagnose it and administer the appropriate treatment. This could be antibiotics and anti-inflammatory medications if it is a hot spot. The second option is to monitor it at home. Use an e-collar (cone) if he is licking or chewing at it. Keep it as clean and dry as possible. You can use a plain, unscented baby wipe or a pet wipe to gently clean it. If it doesn't resolve or causes discomfort, then Caesar will need to got to the vet.
Monitor for rapid change or growth, discharge, bleeding, lethargy or pain associated with the site. These are symptoms that should be addressed by your veterinarian.