Glad he'll be ok with some time and meds. I had something probably similar under my right elbow right after I broke the radial head and ulna at the joint. I had a big floppy liquid filled pouch bump under it, probably leaking serum and things from the breaks. It didn't hurt but was kinda scary but it slowly went away on its own as everything got re absorbed I guess. Dr weren't concerned as far as I remember and said it happens on nasty breaks. Sounds like he did something similar without breaking anything so he should be fine with no issues in time.