Is he in a qt? Watch it for a day. If it's brook, it moves fast. I would gather meds in case and be ready to treat.
Formalin
Acriflavine
Metroplex
They all treat brook but formalin and acriflavine being the go to for brook. If it starts to worsen and you don't have meds, a freshwater dip will provide relief.
Hopefully it's just lympho and stress related. It's just hard to tell from the pic and being it's only a few spots.