As best I can tell, he speaks at the level of someone with about a fifth-grade education. I believe that to be intentional, as it means he's easily understood and not perceived as demeaning.

More importantly, his speech is consistent and has been his entire political career.

Biden's problem isn't that he's not able to speak at a collegiate level; it's that he's very obviously getting worse over time. The man is currently President of the USA - when's the last time you heard him speak publicly and take questions?