My dog has a dark red spot. Could it be a bruise or infection?

That redness could indicate a bacterial infection. Sometimes the infection is a primary problem but sometimes it can indicate an allergy or allergic reaction. Less commonly, skin mites can look like this.
On the right side, there is a very dark red discoloration. This could be present if he is licking or scratching at it. The concern I have is if that red discoloration is bruising. Bruising is not normal in dogs and is considered indicative of a possible emergency. Reasons for bruising include an autoimmune disorder, eating rat poison, a bleeding disorder or liver disease. If you suspect that is a bruise, I recommend to have Jax seen by the veterinarian quickly.
A skin rash is not an emergency. You need to keep him from licking the area by making him wear an elizabethan collar at all times. You can wash the area daily with hand soap and rinse well. You can apply a dab of coconut oil and rub it in well a couple times a day. If it is not much better with in a week, take him to his veterinarian.
If the area gets worse or if he seems uncomfortable, also get him in sooner to be seen.
Lastly, you can always just have him seen by the veterinarian. They will be able to provide stronger medication that will clear that up much faster.
I hope this is helpful and please feel free to post any additional questions.