Red and puffy eyes are worrisome, you should take him to the ER right now.
Allergies are unpredictable, a person can be allergic to something for a long time and then the reactions just lessen or disappear without explanation. It works the other way, too, sudden allergic reactions can happen at any time, and you have to be vigilant with the ones that cause anything to swell, like eyes and other mucus membranes. In children, this type of reaction can escalate to anaphalaxis quickly, it's always best to take them in to be seen.
My daughter started developing eczema around the age of 10mos. Severe rashes were only the beginning. We've taken her to the ER at least 3-4 times with face swelling and never figured out what it was from. She has had many different food sensitivities over the years and we have to keep switching her meds and vitamin supplements every year or so, trying to find the appropriate combination to keep her from having reactions. So, I've been through this, she's 12 now, and we are still dealing with it. I hope you find a simple explanation and are able to avoid the culprit in the future, but I have a feeling this may simply be one reaction in a long line of others. Watch what kinds of foods you introduce from now on, you may not want to introduce anything new for a month or so.