Bald Spot

I cut Owen's hair 5 weeks ago, and there was a quarter size bald spot on the back of his head. I thought I made a mistake with the trimmer, and gave him a bald spot. Well, the bald spot hasn't improved at all. It is still just as bald. There is some hairs, but it is mostly bald. I am starting to wonder if I should see a dermatologist to see if something else is going on.

The bald spot is 2 cm by 2.5 cm.