Whats causing my sons spots!?

My son is 7 years old. He has been coming out in spots which look like itchy bumps, usually only 3 - 10 at a time over a period of 2-3 weeks. They disappear completely then reappear anything from 4 - 7 months later! This has been happening for the last 3 years. Doctors that i have seen first thought it was an allergy, then they told us as we have a dog it must be flea bites - we subsequently had the entire house treated professionally and ensure dog and carpets done every 4-6 weeks. He has recently come out in them again and we were referred to a dermatologist, he said they were definitely bites from an insect, but not fleas or bedbugs as he would be having them all the time from an infestation and not having these intervals of months between outbreaks. I was told its nothing to be overly concerned about, and that although irritating they are not infected or harmful to him. I am at a loss as to where he is getting these bites from and even though they are not harmful i would really like to find the cause so i can have a chance to hopefully put a atop to it! Has anyone else experienced something similar? (nobody else in house or family has had any, and the house was rechecked for any sort of infestation and none were found)