Those look like hives (as a hive sufferer & a mother of a hive sufferer). Save yourself the trouble down the line & while it's fresh in your mind, figure out what he came in contact with that was new to him. I.e.new food, new soap, new clothing, etc. If nothing was new, write down anything he came in contact with period. Might sound strange, but if you make a list & save it, if he comes down with them again (& if they are hives, he likely will), then you can compare your lists to what he is in contact with at that time & hopefully find your culprit. It took my parents YEARS to figure out I was allergic to a certain laundry detergent, a certain carpet cleaner & a certain clothing material (amongst other things I've figured out on my own as an adult). With my son he developed his first case of hives at 10 months after starting amoxicillin. So the doctor labeled him with a penicillin allergy. When he started coming down with them again & again, we pinpointed it to red dye. I'm sure there is more we haven't found out yet though.
All that being said, hopefully it was not an actual allergy. May have just been an irritation!