Yes, indeed he has to stay on-topic in his writing. Some might call this developing a filter, so that all his thoughts are not blurted out... but rather are filtered through one or more mental questions, which he silently asks himself, such as: is this on-topic, how does this add to the current theme, who is the audience, what are some likely reactions/responses of the audience, etc.
Meanwhile, it may be possible to examine the works and sketch a brief rubric. This might begin with a matrix or spreadsheet consisting of 5 or so criteria, each given a value of 0-5 points (or 1-4, or 1-5, etc). For example:
- grammar/readability/use of full sentences (scale of 1-5 based on presence/absence of errors, style, etc)
- proper APA citation format (scale of 1-5 based on fidelity to APA format; Purdue Owl is one well-known authoritative resource for citation information)
- number of resources cited (scale of 1-5 depending on number of resources)
- development of theme (scale of 1-5 based on degree of coherency)
- original thought/analysis/synthesis/connections/conclusions (scale of 1-5 based on depth of insight articulated)
Something like this could be a simple checklist for your son to utilize as he writes, guiding him in his work. He may soon become adept in making up his own structures to guide himself.
After developing a sample rubric for this particular assignment which both you and your son see as reasonable, it may be possible to approach the teacher with the idea of a rubric as a "communication tool" to efficiently state expectations, as well as ease her grading/feedback burden (the time it takes for her to grade and provide feedback). The idea would not be to push for use of your particular rubric, but to offer it as an exemplar which she may adopt/adapt.
The rubric may also help the teacher keep the significance of your son's off-topic sentence in perspective; A 15 point deduction may have been disproportionate and harsh.
