When your kid is acting off and not themselves but isn't having any major symptoms, what do you do? How long do you wait?

F has been off for almost 3 weeks. He's fussy and clingy, which is completely unlike him. He's normally my social, full of life little guy and this last week, everyone kept asking me if E was my happy, social twin. He has had a snotty nose, but no fever or other major symptoms. His sleep is fine. Eating hasn't been great, but he is eating and drinking some.

At what point do I give up and take him to the doctor just to make sure there's no lurking ear infection, etc?