A 19 point drop is a bit steep so I would probably investigate a bit and get more details from the school. In general, I would not be worried if the drop is only 5 to 10 points. If you notice, the standard error (+/-) is 6 to 8 points anyhow. It is possible, as you suspect, that his skills also could have gotten a bit rusty over the summer as well. However, with a drop of 19 points, there may be more at play here. Sometimes testing conditions may not be ideal, so perhaps your DS was not able to do his best that particular day.
Our school tests fall, winter and spring so you can really compare data over time. In your case, I would be less worried if the 221 was a huge jump from the other scores and more worried if the 221 fell within the pattern of scores. For example, my DD's math score last year dropped 5 points from fall to winter but then rose 12 points from winter to spring for a net RIT growth of 7 during the year.
