it's so terribly difficult when they just can't tell us what is wrong

Go with your gut feeling. If you think you need to take him to hossy, then do that. He may have a sore throat? Oskar was not much older and got a sore throat which ended up turning into bronchialitis which was not fun. They are little and a simple sore throat can progress to more faster than with us adults. I hope that he is ok - let us know how he goes.