He's just showing growing pains. I think he'll be fine, just like Conan was fine after a few years. Remember, it took Conan 4 years to really buckle down and get good.
Fallon just needs to work on his delivery. He's obviously not the student of comedy that Conan is, but he's a young guy that knows what makes most other young guys laugh, for the most part. Just needs a moment that really makes his show click, that's all.
It's pretty incredible how much he's gotten better in the span of 4 months though, even if he's not great yet.