What age, breed and weight is your dog?
How long have you seen and felt this?
is is soft and fluid filled or hard?
Irritated, red, sore, etc?
This can be something as simple as a Hygroma, These are common in large dogs, which you can read about here:
However, it can also be a more serious issue, like an abscess, cyst, papilloma, lymph gland or even a malignancy.
As in humans, any lump should be evaluated by a Doctor. Please err on the side of caution and have your DVM take a look at this ASAP. Hopefully it is not serious and can be easily treated. At least you will have peace of mind once you know what you are dealing with.
Please let me know what is found and how your boy is doing.