He should at the very least see a urologist. If it hasn't gone down after many hours it might be appropriate to go to the ER, but he needs a full checkup from a regular doctor who specializes in this, not an ER doc.
What is Priapism?
Priapism is an erection that lasts far longer than normal—often for four hours or more. It may start as a regular erection when you’re sexually aroused. Or, it can pop up without any stimulation. The erection can last for several hours, and can be very painful. If priapism isn’t treated, it can cause erectile dysfunction and scarring. Priapism is most common in men ages 5 to 10 and 20 to 50, according to Cleveland Clinic.
Hi it can be very painful may i suggest you go to a Emergency Room ASAP
it can do i lot of damage if not treated , Take care ,