If the specimen is complete enough, a key will do the job. To use a key you typically need a sample of leaves, stems and flowers. If you're trying to ID herbaceous plants, a key many people like is in Newcomb's Guide to Wildflowers, although the leaves described in his key are basal leaves, not the ones up near the flowers, so you'd either have to have some lower leaves or remember what they looked like. There are plant keys online -- google to find some; I'd use a search term like "PA plant key" and see what turns up. I prefer books, though, because it's easier to cruise around comparing similar plants.

Soeur

See if your local library or second hand book shop has a copy of Newcombs wildflowers. It has a pale yellow cover (unless there is a new editions). It has a simple key structure at the front that deises a 3 -digit # from;
The # of petals
Leaf placement
Shape of leaves.
Then, you go to that number, and it refines a bit with questions about height, and flower colour. Then, it sends you to a page wit 8 -10 drawings to choose from. It is easy to use, and teaches observation very well. Then, consider getting your son a small digital camera, and making him do the posts! He'd have fun with this, I think.

Newcomb's is good, and I'm getting better at using it, but I find that a fair amount of the time I'm not successful with it, especially when the plant is not in flower yet. You may also want to get the Peterson's guide. Don't know if you and your son will have the same experience or not. Anyway, in case you want them here are a couple of sites that might help.

Conn. Botanical Society Wildflower gallery
http://www.ct-botanical-society.org/galleries/galleryindex.html

Virginia Tech Dendrology page for Penn woody plants
http://www.cnr.vt.edu/dendro/dendrology/map/pa.cfm

Lady Bird Johnson Wildflower Center Search. You can specify state
http://www.wildflower2.org/NPIN/Plants/plant.html

Good luck.

it really helps to be able to identify the plant family - with experience he'll be able to tell right off for most of them - is it in the mint family, carrot family, legume, whatever.

Stella, Many years ago I too collected leaves for id of plants on my property, and much later found info on making a proper herbarium. Wish I'd had the internet then to make my task simpler. Anyway, just in case your son might be interested, I'm attaching a link. Sounds like you have a great botanist-in-the-making there! josh

Here is a link that might be useful: Herbarium

I reccommend Newcomb's also. Especially for a science project. There are pages in the back of Newcomb's that explain the nomenclature for leaf and petal identifications.