it would have saved lots of money that didn’t need to be spent
You didn't spend it, nor were you asked to. The people who did spend it were very happy to spend that money to have a professionally arranged, 'fancy', 'magnificent' and 'lovely' party (your words). Not everyone wants a DIY party - they might not have the time, they might just not enjoy doing that.
if we all banded together like I wanted
For the love of god, OP, and for the millionth time - it wasn't your party. No one gives a stuff about 'how you wanted,' they were having their party to their tastes with their friends.
You already stuck your oar in far too much by making additions to the guest list and telling everyone to fill up on food before attending.
Tey didn't tell you details because your input was not just unneeded, it was actively unhelpful given the party they intended to hold.
@Whoisunreasonable - My wedding was entirely DIY, and my friends mucked in enormously. I loved it. It suited our lifestyle and income; we had a great time.
My much younger brother's wedding was at a stately home with a harpist, a band in the evening and designer clothes for the wedding party. I had offered to help of there was anything I could do but it wasn't needed and I think they had a really lovely wedding. I was delighted for them.
My DD's idea of a prom dress was a million miles away from my own. So I shut up and supported her.
You have to let go. These choices are';t about you, you don't always know best and you are not the arbiter of How Things Are Done.
You owe your son an apology for overstepping. This is a great opportunity to reflect and improve your future relationship with them both.