What time are they eating lunch and what time are you serving dinner?
"I don’t cook until about 9" 😱why so late?! It's insane to expect anyone to go 8 hours minimum with no food! Potentially 9 hours! That's not at all healthy.
My mum served dinner at 6 and we still got toast or a jam sandwich or crackers and cheese, fruit and cheap biscuits (Rich tea type of thing) when we got in from school.
I've a rake thin 18 year old who needs feeding CONSTANTLY and while there's an element of that being due to her disability (causes elevated metabolism, high muscle mass, pain means burning more calories etc) her friends are mostly the same.
You then mention you're on a tight budget - so am I and so was my mum when I was this age, and so was her mum (and my dads mum).
I'm sure if you posted your budget and what kinda thing you currently buy/cook we could try and help you juggle things to include an after school snack/small meal.
Maybe your sons gf mum knows this and as pp said is trying to alleviate the pressure of her child's being fed at yours too because she knows it would be unfair to expect you to be able to cover this?
I know it's hard but teens do actually need more calories than adults, they're basically on a huge long term growth spurt.
As you don't know them try not to jump to conclusions too, just because they're well off now doesn't mean they always were nor that they didn't come from a poorer background themselves.
"I just feel like she’s providing all the things I can’t and my son wants to spend less and less time at home because he gets ‘treated so well’ at her house." So it's jealousy?
You need to get over yourself! There will be things you do/provide that her family perhaps don't - that might be time, your presence, less rushing about etc.
Different families work in different ways and there are benefits and down sides to all. You need to focus more on the benefits of your situation, but you also need to adjust your mindset on food.
I actually wrote a whole load on eating at friends houses and them eating at mine, I'm an army brat so I've moved around a lot and made friends with children who's families are from lots of different countries/backgrounds/cultures but I got a bit distracted remembering all the lovely (but cheap!) food.
Teens are somewhat nomadic, that's normal, they're making their world bigger and preparing to leave home and coping with being part of the wider world. Add in hormones and romantic fantasies and they can be frustrating little so and so's!
But honestly unless you want to alienate your child you need to be more flexible, more welcoming of people from different backgrounds to you and more accepting of people's kindness.
Being kind and generous yourself is kind of easy and satisfying - that's why you get virtue signalling.
But accepting kindness and generosity is a real skill, and it's good manners to accept graciously.
If it would make you feel better you can return the kindnesses in ways that don't cost you money we all have something to offer. Maybe they have younger kids that need babysitting sometimes or pets that need looking after if they're on holiday or plants that need watered or whatever - we all have something to offer.
I'm housebound with various health issues, my dd has lived with me being ill all her childhood and I remember feeling that twinge of jealousy when dd called her friends mums "cool" (except a different word cos apparently "cool"
Isn't cool any more 😂) cos they took their kids to things I couldn't (either due to health or lack of money or both) or bought them things, but then she's also mentioned to me that her friends thought I was "cool" because I'm very open minded, happy to talk about anything, very into music and still stay up to date on that score, also a huge film fan so good on recommending films they'd like, I'm also normally (I'm starting to struggle now though) quite good on tech stuff and how to quickly do stuff on phones/tablets...
So I had other "skills" to offer.
It sounds like you've had a tough time recently - give yourself a break on that score, but yes you need to adjust mealtimes/providing enough sustenance.