I know how you feel. My son loved Dora too.
The only things that I can think of is going with the purple decorations ( boys like purple too) if they arent light "girlie" purple. Are you talking about plates and stuff like that that has their faces on them? How about the purple plates and then buy another color napkins, cups, streamer...... Boots wears red boots, I think? Maybe focus on that. I would try to focus on Dora, Boots and Swiper all together that way it could be for a boy or girl.
You could do some sort of invitation with "Help Dora and Boots keep ....(sons name)....away from Swiper. Join us for our adventure". Have a treasure hunt or something along the lines of the show. Dont forget to sing the theme song!
This could be really fun.

Post back when you finalize your plans. Most of all, have fun with him!!!
Good Luck!
Tami