I take it you are a permanent resident filing for your son who is under 21? That would put him in the family 2A category.
Historically, that category has had a wait time of 4-5 years (except for Mexico/Philippines with a much longer wait). But in 2010, it made several large jumps forward until that dropped to under 6 months. The stated reason for those jumps was that a large number of eligible applicants were not showing up for the final stage (to apply for adjustment of status, or for an immigrant visa through a consulate). So they moved the dates forward to make those visa numbers available to people who filed more recently, as the old filers had apparently abandoned their cases.
But the category recently reverted to 4 years which is in line with the historical norms. I figure it is because those people who weren't showing up throughout most of last year had suddenly started filing for AOS or immigrant visas.
