Ditto on what the other moms suggested...and here's a few more...
Call the United Way...211. They have all these resources! They can even get you in touch with other organizations that may help you with your holiday meals and gifts. I know they helped me a few years ago...
Along with Craigslist, there's also Freecycle.org...local chapters, etc, and everythiing is FREE! Do a search on Twin Cities and Free stuff...you'll find a lot of resources there. www.twincitiesfreemarket.org is another one that's good.
You're lucky... at 3-4 years old, they usually don't have much of a concept of how much something cost...AND, its a great time to teach him that it's the thought that counts and the true meaning of Christmas. I know one family that wasn't doing well, and in the Spirit of the Holiday (and their budget), they only gave three presents.
That being said, I remember one great Christmas, my boys were 2 & 4. I went to the dollar store, spent about $20 on silly little things and coloring books and crayons. I wrapped everything individually, and they had the best time! Their "big" presents (approx. $10 each) were hidden and we played hot/cold to find them. I got them a game that they both could play (the fishing one, I think, and we still play with it!) and a flimsy racetrack. I think that Christmas was better and happier than the one last year when I had money and got them nearly everything they'd asked for...Wii included! (Now, why did I do that?!?) :)
Look around at the toys he has to see if there's anything to make him excited about something he already has. I guess I'm thinking about a racetrack...if he gets a new car, he'll play with it again like it's new. ($15 - 20 for the racetrack, $2 for 2 cars.) I have to admit I've done this for a couple of years, now...and it still works! :)
Another thing is to get his outgrown clothes and toys to Once Upon A Child...you can either get cash (or check) back to spend somewhere else, or get him some toys there.
One last thing. The stuff he sees on TV that he wants that costs more than $20 will more likely than not be played with for 15 minutes and tossed aside for an empty box or the ribbons off the presents. :) We joked around that this year, we're getting them both empty big boxes and plastic grocery bags. They were hiding in the boxes, and running around making the bags fly for almost an hour (at 5 & 7 yo!).
Good luck...and have a wonderful Christmas AND Birthday...it'll be wonderful for him regardless if he can have time playing with you!