First of all Dawn welcome to the forum!
What a horrible vet they have no right to make you feel awful like that
Right you will hear a lot of bad things about Wagg and that's mainly because it contains cereal and meat derivatives and lots of people think that this makes the food inadequate and the dog will be hyper or it'll have digestive issues because dogs can't digest gluten and they should always have a large percentage of meat and its like feeding your kids exclusively on McDonalds and all that - I'm guessing that's pretty much what's upset you today. Well to be honest lets see all the people making those statements come up with the clinical trials that prove that is the case, I tell you what I bet they can't. Just like with humans there are lots of people who get very het up and blame everything in the world on something and they have proof and emotions run high and this worked for their dog their friend's dog all the dogs in the world, dogs can't digest milk, gluten, cereal etc etc add evangelical raw/barf feeders into the mix and they'd have you believe all of us who don't raw feed are evil swines killing our dogs with every mouthful! Its a complete load of nonsense and actually in the case of many claims scientifically proven to be untrue and of course they never tell you the downsides. It's no wonder people are confused dog food companies with their claims, owners with theirs, vets with another, it's enough to make your head spin.
I can categorically tell you now without even meeting you that you are not a terrible dog owner and do you know how I know that, because you were at the vets and because you got upset if you were some terrible person who didn't care you wouldn't have been there and you wouldn't feel bad so take a deep breath put aside all this doubt and lets see what we can do.
It sounds like something has changed for Charlie since this has just developed and that could be that he has allergies given the time of year maybe to pollens so he's liked the fur off, or to food or treats - have you changed them, washing power, air freshner - all sorts of possibilities. Alternatively he could be in pain and pulling the fur out due to that. He may have a deficiency or something malfunctioning but your vet can do bloods to rule out things like thyroid problems so instead of having a go at you ask them to get on with it. If you haven't changed food recently and yet this is a new medical issue it is unlikely to have been caused by the food.
For any dog, and even more so for those who have delicate digestion, one size does not fit all, no one can tell you the one food that will work for all dogs because one doesn't exist. Just because something works for someone else doesn't mean it will work for Charlie so if you find a food that he is happy and healthy on for goodness sake don't change it! If you start making changes when your dog is fine or if you keep swapping this that and the other because well intentioned people gave you advice that was right for them but didn't work for your dog then you can cause tummy problems that take ages to resolve.
Please note vets often sell dog foods and go on courses funded by dog food companies so their advice on brands to choose is somewhat biased!
Personally I don't know a single person (and I know lots of dog owners) that has found James Wellbeloved (JWB) to be any good even if they do have a lovely ingredients list a nice helpline pretty packaging and heaps of promises. Everyone I know found their dogs were windy and produced lots of poo not exactly the selling points the company was going for! Remember any company you ring will tell you their food is ace, perfect for your dog, have a free sample, call anytime, blah blah blah, of course they do its called marketing they don't really give a flying pig's ear.
Personally I feed original tinned chappie it's really cheap its easy to buy and yes its made with derivatives and yes it contains gluten but you know what it suits MY dogs and that is all that matters to me, if it stops suiting them I will stop feeding it. It is a good food that lots of the old vets recommend for weight gain, weight loss, sensitive tums, liver kidney pancreas issues all sorts no one knows why it works sense suggests it shouldn't but it seems easy to tolerate and digest and sometimes that is just worth accepting. I don't say that because I'm a cheapskate either on my mission to be the best owner/parent I could be I have driven myself crazy on what was best fed Orijen, Acana, Barking Heads, specialist vet diets at £50 for a tiny bag that barely lasts a week and I didn't care I would give them anything if they needed it and if it had worked they'd still be eating that. What will be will be.
If whatever you feed your dogs keeps them happy and healthy stick to it and ignore anyone who tells you otherwise.