Hey, there! It actually looks like you're looking at a verb tense agreement error. Subject-verb agreement errors have to do with the *number* of both subject and verb agreeing (for example, if it had said "will they goes with us" you'd know that was a subject-verb agreement error because "goes" is a singular verb and "they" is plural).
The problem in this sentence, though, is that "goes" is the present tense form of "go," but "will" indicates that the question should use the future tense. So, "will he go to the lake with us on Saturday?" agrees on verb tense while "will he goes to the lake with us on Saturday" does not.
Let me know if you have any other questions!