I'm sad to hear about your problems.
Switching a ball python from eating live to F/T is a lot of work, especially for someone who hasn't had a ball python before. I am also a bit concerned about him still eating pinkies. He's not a hatchling anymore. I think he should be on adult mice by now but if he's not big enough then that's not possible.
Honestly, I think the breeder you chose to go with is not a good breeder.
Do you have a photographs of the little noodle? If yes, could you post it?
How serious is his condition?
If he's loosing weight, then there's a problem. Have you weighed him and kept track of his weight?
How long can a 7 month old ball python go without eating?
If he's not loosing weight and just simply fasting, then he can go for months without a problem. There are cases in which ball python fast for more than 9 months.
Why was he on pinkies?
I'd like to know that as well.
Why wasn't he able to get the hopper down?
The animal seems really stressed to me. Sometimes switching to bigger sizes can cause problems for ball python. If the hopper is a big as the biggest part of the ball pythons body, then he should be able to swallow it. If not, then you must choose a bigger or a smaller prey item. For now sticking with pinkies (maybe take 2 when you feed him) is probably your best option.
Why was he dehydrated?
I believe the breeder isn't a very good one.
What do I do now?
Try not to stress the little worm and give him some time. Change his water often and take care of humidity. Try to feed him every 5 days with pinkies for now and keep track of his weight.
I don't want to cause him any more stress than necessary so do I assist feed again?
Assist feeding is very stressful for a ball python. I would suggest not to do it unless there really isn't any other way.
When do I offer the hopper?
When he has eaten the pinkies for about 5 times in a row and looks healthy to you.
I hope this was a least a bit helpful. There are a lot of people in this forum who lots of experience (way more than I do). The best advice I can give you is to listen to their advice!