There was a section in the Disney Family Fun magazine about some games that were really cute.
The one was to take a tissue box and put a belt through the bottom of the sides so it could be worn around the waist. Then put some ping pong balls inside and see who can shake all of the balls out the fastest. You could use string or ribbon instead of belts if you don't have any handy.
Another was to take shower caps and put shaving cream on top and have a partner *from a short distance away* throw cheese doodles (popcorn would work too and be cheaper). Object was to see which team could get the most on the cap.
They also had a ping pong ball throw set up with cups on a table and then you had to see who could get the balls in the cups.
I think I've seen ping pong balls at dollar stores...
Also, my mom always had balloon pops and we loved them! You make two teams and have two chairs and enough balloons to spell out something like "Happy Birthday!" for each team. If that is too difficult, make it your son's name. In each balloon, put a slip of paper that can be found when the balloon is popped (it's popped by sitting on it in the chair). Some of the papers should have the needed letters and some of the papers should have something else (Try Again, :(, a symbol like a star, etc.). The goal is that the team pops all the balloons and then works together to spell the message to win.
You could also have a pinata and that can be cheap if you make it. You use an inflated balloon and a glue and water mixture with newspaper strips. (google for exact instructions) Paint or leave the way it is if you use the comic section....fill with candy and duct tape the small opening where you put the candy in at. Hang and enjoy! I loved making a few pinatas with my mom when I was younger and can't wait to make one with my daughter someday!
Sack races with old pillow cases would be fun too!
Also, you could have a relay race with small water balloons on spoons...Make sure to have extra balloons on hand for when they burst so they can keep going! LOVED that one when I was little!
Just some suggestions. Hope they help....