I'm not sure if you received my last question or not.
The first thing I think of when I hear of a lump that is growing that quickly is an infection.
That can be from a bite or scratch wound or a foreign body puncture wound (stick, grass awn etc). Because he has buddies he plays with that makes infection even higher on my list. If you have a localized, walled off infection then the pup may not be febrile and may behave fairly normally and they can feel pretty firm as they are under pressure.
If the lump is in the area of a lymph node then we can see lymph node enlargement secondary to infections, both systemic and local, or trauma. If he were a bit older and multiple lymph nodes were increased in size then Lymphoma, a type of cancer would be possible.
If the swelling is in the area of the mandibular lymph node another thought would be a salivary gland infection or blocked duct. Usually a blocked duct will produce a swelling that feels a little softer then you are describing but a salivary gland infection or inflammation can feel quite firm.
Since he seems pretty much himself I'm not really concerned yet.
I do recommend looking in his mouth to look for any signs of broken teeth, a stuck foreign body or gum infection.
I would also look at the lump itself to see if you can see any evidence of a small puncture wound. You can warm compress the lump for 10 minutes at a time several times a day, as if it is an abscess getting it to break open and drain would be ideal.
Your veterinarian will likely want to aspirate the swelling, and look at the collected debris to see what is causing it. If it does look infected then lancing it and putting him on antibiotics will be recommended. Given that he may need to be sedated do not feed him after midnight today so he has an empty stomach for his appointment tomorrow.
Please let me know if you have any further questions.