From what you have described, this is no big deal.
It could be a gland reacting to an infection or indeed it could be a salivary gland blocked by a stone.
The latter is unlikely.
Monitor the situation over the weekend and if he starts to run a temp then give him Ibuprofen. In this age groups Glandular Fever comes into the frame,but the treatment is just the same as i outlined above.
Monitor the situation.