|Unfortunately poison ivy, oak, or sumac stay potent all year long so there is no "season." They are perennial woody plants that lose their leaves in winter BUT can still cause a rash from the bare branches, vines, or roots. The resulting rash looks the same from all three types of urushiol containing plants. In spring they will sprout new leaves on established plants, or new sprouts from seeds and underground shoots.
Since your son is so sensitive I would limit his time in the woods. Wearing clothes to cover up is great but the oil can even penetrate rubber. That is why it is recommended to wear vinyl gloves when working around PI. He is old enough that you can teach him to spot poison ivy or oak. Kids learn fast and I'm sure he hates getting it year after year.
One thing you could make a habit of, is anytime he has been in or near a wooded area, make sure he washes immediately with cold water and a soap like Fels Naptha (a laundry bar soap) to strip any oil off his skin. Burt's Bees makes a poison ivy soap. Wiping down exposed areas with alcohol and then rinsing with lots of cold water can remove PI oil.
IMPORTANT! Never give him a bath or use hot water if you suspicion he has been in contact with PI as this will cause any oil on the body to spread to other areas. If he needs to bathe his entire body, then a cool SHOWER with the special soap. If you can catch it in time you won't break out--but research says the oil bonds with the skin proteins in the first 20 min. or so of exposure, sometimes faster. I have used Fels Naptha for years and it works great for removing plant saps from your skin. The time frame is what is important here...what too long and it won't matter what kind of soap you use because the damage will already be done.
Did your son get his rash infected from scratching? I feel so sorry for him. I've found that very cold cloths (wet but not dripping) were soothing and help with itching, oozing and swelling and is a safe method for relief. Ibuprofen can help with pain & swelling also (like children's Motrin). You did not say if he was on any meds from a doctor but I assume he is if he has an infection.
Take care and I hope he feels better soon.