My husband gets poison ivy by just looking at it, I think! Luckily, I'm one that is not allergic to it so I've never had it. So far none of our children (ages 5, 2 and baby) have gotten it. We're very careful because we do have some growing near the creek at the back of our property and also in a couple patches at our lakehouse.
My husband uses Tecnu ALOT. The pharmacist has told us it's safe for the children to use as long as we rinse it thoroughly -- which you do anyways to remove the oils from the poison ivy.
A few things we've learned about poison ivy... the oil stays on things for a very long time. If you're allergic to it and touch the oil on something (garden tools, clothes, shoes, even the dog's fur if he's been near it) then you'll get it. It can stay on dead and/or dormant poison ivy plants for months, too. Many people get it during the winter without knowing because they touch a dead branch. Since there are no leaves, they can't tell. After being in an area of poison ivy, my husband uses technu to wash all the tools and shoes. He does a load of laundry with just those clothes and washes them with technu. He also takes a shower using technu. Since being so careful, he hasn't gotten it for 2 years.
As for the pants. I know it's hot, but there are special light-colored lightweight pants you can buy. Look at a specialty hiking/backpacking store. He should be wearing pants in the woods anyways because of ticks and the high threat of lyme disease in this part of the country. Anyways... hiking pants work great. A long sleeve shirt should be worn, too, if he's that allergic to poison ivy. REI has a big variety of that type of thing for children. Then just make sure he strips down and puts those clothes in the laundry as soon as he comes inside. And use a pair of shoes specifically for being in the woods and not worn at other times.