I don't even remember Indiana Jones very well, but a couple resource and other ideas.
I did a Google search on Indiana Jones birthday party and found:
http://www.questexperiences.com/quest2/portstoadventure/i... - on e-book for $9.95 that promises hundreds of ideas. Might be worth it.
Found this article on what was done for an older man's ideas - might spur some ideas (I liked their invitation idea!):
Kids LOVE scavenger hunts - do a scavenger hunt and at the end have little treasure boxes with their party favors. You can get them anywhere - party stores, etc.
Try orientaltrading.com - they have so much really cheap stuff, from party supplies to favors to craft ideas. Search jungle - I found a bunch of stuff. Not necessarily exactly right, but you might find something. I bet they might even have some kind of jungle vines you could buy to decorate with. I know they have blow up palm trees.
Dress your son up as Indiana Jones - get him the khakis, hat, a pretend whip, etc.
You probably can buy some Indiana Jones posters to decorate the party. Then your son can hang them in his room. Here's an Indiana Jones shop online - http://shop.indianajones.com/?rid=CPC
Oh, and you can use the poster to play "Pin the Fedora on Indiana Jones" or something like that. For my son's Seattle Mariner's party, we had a poster made and laminated of Kenji Johjima (his favorite!) and made little baseballs with each child's name on them and they played "Pin the Baseball on Kenji."
Get fedoras for all the guests.
Get a CD of the theme song and play it at the party.
The web is full of ideas and resources if you are creative about how you look. I'm sure you've already done a bunch of that though.
Just some quick thoughts - I have to get back to work!