Also, your DS should look for meaningful opportunities to fill his summer (job, volunteer service, etc.) and to bolster his leadership experience at the start of the school year (team captain, officer of a club, etc.). IMHO, I would suggest working on essays and other supporting materials over the summer, but wait until after the beginning of the school year to make sure his package is strong as possible when submitted.
Note: all three of my DS's MOCs had different submission requirements. Make sure your DS reviews requirements and deadlines for each potential nomination source.