Selling candy bars is labor intensive for little money. You charge, what, $1 a bar for something you pay 50 cents for? So it's 20 bars to make $10? That's a lot of door knocking. He's going to have to cover 4400 homes of people who actually purchase a candy bar (not counting those who aren't home or who say "no") to make $2200. Is that worth it? Is that practical? And you definitely need some kind of paperwork that shows he's raising money for something legitimate - school trip - because people get their doorbells rung for this stuff all the time.
People will pay more for a real benefit. That means mowing lawns, doing yard care and plant watering for people at work on on vacation, pet care (same thing - people away), dog walking, weeding, washing cars, and so on. My son had a 10 year career from age 8 to age 18, doing all that stuff. That means he was considered trustworthy and he had keys to about a dozen houses in the neighborhood - he had some regular jobs (lawns) and a lot of occasional jobs (vacation coverage) that included everything from bring in mail and packages and putting out trash every week to emptying water from dehumidifiers (maybe not a big problem in NM where you are!), to watering plants while people were away. He had to be flexible and to grab every job whenever it came up. He fed lizards and guinea pigs, cleaned fish tanks, walked dogs, fed cats, picked up poop, and gave lonely animals some need socialization.
The great part was that he learned to schedule jobs, to estimate, and to invoice people who weren't home when he did the work. So he learned some basic accounting. All of this went on his "activity resume" that went with college applications, and he had some great references from people he had worked for over a period of years.
It means your kid has to get up early and be ready to work too. He can also get a part time job bagging groceries or re-stocking grocery shelves. I'm not sure how old he is - you have to be a certain age to run the deli counter equipment, but other jobs are available for 15 and 16 year olds.
These jobs require effort but no cash outlay, and it shows more resourcefulness and requires more skill than selling candy bars.
Your son can also work at a farm stand, an ice cream shop, a fast food place, and so on. Not glamourous. But he will learn the value of a dollar, and he will learn that working for what he wants is the way the world operates.
I don't see why he can't have a part time job through the year. Calculate how many hours he spends doing stuff that isn't productive - most kids are in front of a screen or just hanging around a good portion of the time.
Do some basic math with him - if he earned $8 an hour, how many hours does he have to work to make $2200? How many hours a week is that? And open a bank account for him - most banks have no-fee student accounts. But he has to be willing to stockpile that money and not get all excited about a balance building up, and decide to spend it on an iPhone.
I think $2200 is a high-priced trip and you should find out how many fundraisers the school is planning and how that money is allocated to different students (is it to the entire program, or is it based on the hours a kid puts in on the fundraiser?).