3.5 yr son stung by bee 4x behind his ear. Mild swelling of sting site. Gave Benadryl (diphenhydramine). 2 hrs later red swollen line runs down middle of his forehead?

Hypersensivity. Your son is likely having a hypersensitivity or "allergic" reaction. Benadryl (diphenhydramine) is an appropriate medicine to give. Sometimes, bee stings can result in anaphlyaxis, which manifests as shortness of breath or wheezing shortly after the sting. See a healthcare provider if any your sons symptoms increase in intensity.
Get in. I would recommend that you get your son in to see a physician. The increased redness may be the beginning of an infection or progressing reaction.