Hi!
Please see a vet tomorrow. It could be an abscess from a bite since he is right in the thick of the teenage months (I assume he has a same age mate?). He will need an antibiotic and may need to have it drained and keep it drained; if the abscess is already starting to swell up quickly, you need to see a vet promptly. Lumps can come up very quickly unfortunately.
Please contact us for further advice on possible support care in addition to medical care when you know what it is. We cannot give you a diagnosis without a hands-on examination in the case of lumps.