If he got into it that long ago, it isn't still spreading. The new spots you are seeing are places where he got so little of it, it's taking til now to show a reaction. Does that make sense?
Your son has my sympathy. I'm one of those people that break out if they just look at the stuff. My husband is immune to poison ivy, doesn't break out no matter what he does with it. Then I get an outbreak from handling his clothes in the laundry!
One thing that may help with the itching, believe it or not, is ice. When a spot itches, instead of scratching, rub the area with an ice cube. At first, you feel like "Why am I doing this?" Keep it up, and after about 20 seconds, the itch stops!