Get to a vet, you idiot.
He got an erection and ejaculated but the semen congealed around the penis and prevented it from re-sheathing. Guinea pig semen sets like Super Glue.
If he is screaming when he takes a piss, this is why. The flow is restricted because semen is clamped around his shaft.
If you don’t want to take him to a vet, wash his dick with warm, soapy water and try to break off the hardened semen ring. It is not soluable, so don’t wait for it to melt.
This is better done by four hands, two to hold the pig, who will be going apeshit, and two to wash his pecker.
If you don’t do something his dick will become infected and you’ll have to give him an antibiotic.