First of all, just wait a little while and see what happens. These episodes seem to be very common and seem to have a lot to do with the begining of the separation process. They usually pass within 24-36 hours. If not, then it's time to get help. If it is the begining of the separation process, he may have several episodes over the next several years. No attention is needed for these occurances.
The foreskin is a very sensitive area and will very quickly get red and even swell at the slightest provocation. It will just as quickly return to normal.
If it's not part of the separation process, it will last longer and needs to be examined. The doctor will first do a visual examination and determine if it is bacterial or yeast and treat it as such. If that does not resolve the problem, then a culture will be done. Retraction is not necessary for a culture sample and the doctor should be reminded not to retract.
The citrus could be the problem if he has very sensitive skin but probably not. Just as a precaution you may want to discontinue the Clementines for now and try them again in a few days after the problem has been resolved. The problem going away does not necessarily mean the citrus was the problem. It may just be that his body is doing exactly what it should be doing, repairing itself.