Tiny, wingless insects, usually tan to grayish, that crawl close to the scalp. Their eggs (nits) attach firmly to individual hairs.
A small, dark, flattened insect that jumps. Fleas are more commonly found on pets or in the surrounding environment than living in human hair.
A tiny arachnid that attaches to skin and feeds on blood. Check whether it is attached to the scalp rather than simply crawling through the hair.
Can you upload a clear photo of the insect? A close-up image, ideally next to a ruler for scale, would help me narrow down what it might be.
In the meantime, avoid applying insecticides or household chemicals to your son’s hair until you know what it is. If an insect is firmly attached to his scalp, it may need different handling than a head louse.