Be sure to continue washing the glans of his p***s and under his f******n to prevent infection. Unfortunately, he is at an age where his genitals are very sensitive to any touch, and he will get erect very easily, as you have learned. Since you need to continue to wash him, try to be very gentle when washing his genitals, especially his p***s. Boys that have not gone through puberty yet, as I would imagine he has not, have dry orgasms, where either nothing comes out of the p***s, or just some very minimal clear fluid. The only way to relieve the discomfort is by o****m, but he probably is not capable of ejaculating yet. I understand that his erection may cause him some discomfort, but that is to be expected. When he becomes flaccid again, and eventually he will, the f******n will return over the glans on it's own. When he gets erect, it is best to let it be. Anita, DO NOT try to put your son's f******n back while he is erect, as this may cause the skin to tear and cause your son real problems.
