I would certainly follow-up with the ENT.
If I were the parent, I would be looking for things such as tenderness (do the nodes hurt when touched/squeezed?), are they squishy or hard (this is tough because we're not medically trained to know the difference), do they easily move around?
Those are all signs of healthy lymph node swelling. Nodes are meant to collect and deal with infection and can take a while to go back to a normal size (1-2 cm).
As a lymphoma survivor (Hodgkins), I, unfortunately, know a little more about lymph nodes than the average person. But, I'd make sure to follow your Mommy instincts, and if they're saying to press on, press and don't look back.
Also, if they offer a fine needle biopsy (aspiration), request a lymph node biopsy. Aspirations are often pathologically inconclusive and will require the biopsy anyhow.
Good luck! Hopefully, he's just dealing with a virus that's taking a little more time for his body to process.
Do yourself a favor and stay off the internet for "lymph node" searches unless you're viewing sites such as WebMD, MayoClinic, etc. You'll scare yourself into thinking everything awful is going on.
Here are some good resources for you: