He is made like that. That is his most notable feature about his appearance.
>Brock's most notable feature about his appearance is that his eyes naturally appear closed. The rare times people see them open are when they are forced open unnaturally: from a hit with Charmander's Flamethrower in The Ninja Poké-Showdown, when being hit in the back of the head by Arbok's tail in Who Gets to Keep Togepi?, and when he makes a face for Larvitar in A Crowning Achievement. This has been the center of fan attention for a long time.
Here is a Picture of his eyes Forced Open. In ("Brock with his eyes open in A Crowning Achievement)
I love Weird things ¯(°_o)/¯