What rebuke does Jeth receive when he casually refers to the president as "old Abe"?

In Chapter 4, Jethro's mother is so shaken after reading Tom's letter that she sends Jethro over to spend the evening with Shadrach Yale (Shad).

Shad is the local schoolteacher, and definitely a hero to Jethro. Upon hearing that he may get to spend the night with his favorite teacher, Jethro is ecstatic. He doesn't think too much about walking the mile to Shad's log cabin, which adjoins the schoolroom. When he gets there, Jethro tells Shad that his mother has sent along a loaf of fresh, white bread. Shad is grateful, telling Jeth that the bread will go nicely with the chicken he's roasting.

Shad and Jeth talk about many things, among them, Shad's desire to marry Jenny, Jeth's fourteen year old sister. As Shad is only twenty, Jeth's father isn't too keen on the marriage. Also, Shad is making preparations to go to war. As Shad and Jeth converse, Shad teaches his student about battlefield logistics and warfare. Jeth finds the conversation especially invigorating until the moment he addresses Abraham Lincoln as Ol' Abe. The text tells us that Shad's response is just a quiet "Mr.Lincoln, Jeth."

However, Jeth obviously thinks of this as a rebuke presumably because he feels that his faux pas (blunder) has lowered himself in his idol's estimation. However, like the consummate teacher that he is, Shad merely continues in conversation with Jeth, acting as if Jeth never made the unintended error. Shad never embarrasses Jeth, and we can clearly see why Jeth idolizes Shad.