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Not getting assignments in on time may be motivational in nature, although it could also be underdeveloped executive functions. (I may be a bit biased, because that pretty much describes my English class production at that exact age, and it definitely wasn't because I was unable to meet expectations. In my case, I think it was a combination of motivation and perfectionism.) I am actually more concerned about the errors in written expression. I would be less concerned about careless spelling and punctuation errors, as those may be self-monitoring errors related to ADHD or low engagement, and are also easily corrected on a word processor, but grammar and organization are substantive weaknesses in written expression, especially as you look toward lengthier, more sophisticated writing expectations.
Although I am generally fairly pro-skip, I do think skips should be approached thoughtfully, considering not only rate of learning and level of reasoning and conceptual skills, but also production and work skill (executive functions, IOW) expectations, among other things. In this case, I lean toward Dude's thinking, especially where he has a history of executive function and written expression weaknesses.
But it would certainly be good to test the various hypotheses, in whatever way you can prior to a skip. When motivated (on those assignments he likes), can he produce lengthy, organized, high-quality work in a timely manner? Or are some aspects still lagging even when engaged? Perhaps investigate the meaning of "bored" and "gets along with". Is he in fact being actively bullied right now? Or does he feel excluded? Is he confounding challenge level with social climate (in either direction)? Is he unevenly challenged, so that some areas are below his level, and others are actually difficult to engage with because they draw on his areas of weakness (writing, possibly)? Both may feel subjectively "boring".
