I did this for a while when my kids were younger. We had a themed family night every Friday. First night was a carpet "picnic in the park" in the living room. I just set up a sheet tent and we played games and ate pizza.... but things got more elaborate after that.
I had a "blue" night. I set the table with all blue... paper plates, etc. I put blue food coloring on the foods (rice, green beans, milk, etc). Everyone had to wear blue to the table. I decorated with blue balloons.
Another time we had "nature night" and I cut saplings and such from outside and decorated the dining room to look like a forest.
Then once we had "Africa" night... we decorated the dining room with stuffed animals, I wrapped DD in a large piece of fabric so it looked like she was wearing an African inspired dress. We made one or two African menu items (can't remember what).
One Friday was "Outer Space" night! I hung a globe from the ceiling and decorated the room with twinkle lights to look like stars. I made the kids mini-costumes... DD was an alien and DS was an astronaut. Can't remember what we ate, but dessert was a cake that I decorated with toy astronaut, alien, and space ship and stuck July 4th sparklers in it!
Anyhow.... I'll stop rambling. I LOVE that you want to do something special like this with your family.... and it certainly can be done a budget.
Your DS is young and even a small theme will be exciting for him. Pick a letter and work around that... it will help him learn the letter and will be fun and easy.... "M".... meatloaf, mashed potatoes, macaroni, milk, mango, mints, marshmallows. You could play games with the letter "M".... memory... and your guest of honor could be Mickey Mouse! (oooo... I just thought of that!) Well... you get the idea. Pick a color, number, letter, movie, book, song, or country and work a theme around it. Good luck..................................P