You could see about getting some quicklime, dries it out and tamps down the stink of rot a bit. Also helps counter the effects of decay on the underlying soil ph.
Failing that, depending on how fresh it is, it might be worth finding something sharp to make a few cuts or punctures with, preferably from a bit of a distance so a knife is kind of a last resort unless you have a strong stomach. Gross, but speeds up decomp and minimises the risk of future corpse explosion.
Both methods might make you feel like a serial killer mind you.
How rotted is it that you can't tell what species it was though? I would have thought a hog and a dog were rather distinct even after death. Or is it tangled up in plants?
Maybe some beadwork? Walmart (I know, I know) has a lot of little $1 packets of beads and wire. Along with some $10 kits with tools. There's also that popular brand of colored plastic thingies for making designs, dunno what he's up for at 8.
Maybe Lego? I assume it never went out of style.
Sadly he has no patience for crafting type stuff. He likes lego, but doesn't really like to build it, and he's already getting a bunch.
It doesn't help that his birthday is Christmas, so he gets all his gifts on the one day, and gets a lot of them. Makes it harder to work out what he'd actually like when there isn't a gap of a few months for him to shift his interests.