Hello and welcome to the forum. First of all, you are capable of reducing your dog's weight but you need to be realistic. I understand your concern but please calm down and take your time - this is really, really important. A slow, steady weight loss is best and it could take as much as a year to get your dog to a normal weight. I think that you need support and if I were you I would seek out a vet who has a veterinary nurse who runs a weight loss clinic. This can be enormously helpful because the dog will be weighed regularly and it will motivate you. Also, she may be able to advise on a suitable diet.
I am not a nutritionist but have dieted several of my dogs over 40+ years of dog ownership and even now with the three I have it is work in progress as they are speyed bitches. I can therefore only speak from experience.
IMHO mixing foods as you are doing now muddies the water somewhat and I believe that you would get the best results by deciding whether you are going to feed kibble or raw. You will then know what does/doesn't work and the exact quantities that you need to give. The amount you are giving now seems rather high to me. Coaster has helpfully provided an ideal weight for a Cocker (14.5kg) so for a raw only diet at 2% of body weight he should be getting no more than 290g. However, you don't say what kind of raw diet you are giving - some can be high in fat which may not advantageous.
Low fat/high protein:
A few years ago my little dog needed to lose 1kg. At that time raw diet was not trendy. I talked to an adviser at James Wellbeloved and she told me that a diet that is low in fat, high in protein helps in weight control. I used their Light version and added in lean protein such as cooked chicken. IIRC I gave it at a lower amount than the RDA. It took six months but the dog did lose the weight and apart from a blip when she went onto a particular type of raw, she has kept it off. Some people say that it is carbohydrate that puts weight on but I can only speak from my own experience. However, dogs need fat so it's best not to be too low.
A raw diet can help you achieve weight loss but it needs to be the right type of raw (you don't say exactly what you are feeding). My dog seemed OK on Nutriment Light but rapidly gained weight when I transferred her onto the Working variety. I had to reduce it to less than 2% of body weight which did not satisfy her hunger. For some time she had Natures Menu Country Hunter nuggets (which are lower in fat) and I had to increase it over time because she was losing weight. I know that Tinyplanets had the same experience with her dog. This is why I think that if you want to stick with raw then do so but choose carefully. I would go with one of the complete meal companies and seek their advice. Remember that the amount should be set at 2% of the body weight that you want the dog to be.
Eden is a good food but if you decide to stick with it then have a word with the staff. The fat level of Eden is comparatively high at 19.6% and if you can only give small amounts it may not satisfy your dog's hunger so it may be necessary to bulk it up with cooked vegetables. A few years ago I tried two of my dogs on a high quality kibble which was similar to Eden in the analysis and they very quickly put weight on so I learned from that experience that this kind of formula doesn't work for two of my speyed bitches. Arguably, this kind of food is more suitable for young, working, high activity, high energy dogs. However, there are lots of pet dogs who do very well on them. It is a case of getting the quantity right. I may be wrong but maybe 1% of the desired weight might be a good place to start. It can be increased or decreased by 10% stages as required. However, chat to the staff first.
If you decide to use dry food but want to source a light version i.e. low fat, we can help you in this by using the Dog Food Directory. My three are currently having cold pressed food which is the low side of average in fat. With added lean protein their weight control is good.
On the subject of exercise, it is tempting to give a lot of exercise to try to shift the weight but please don't. It sounds as if your dog is struggling so I would urge you not to push him because he is considerably overweight and it might strain his heart and joints. Also, it is not advisable to exercise dogs in this warm weather. Short, frequent walks, perhaps in the morning/evening when it is cool would be more appropriate. When he has got down to a better weight he will feel equal to exercise and that will make the task easier and more enjoyable for you both.
Please post back with your thoughts about the advice we have given. Many people do not respond to their threads but we are always interested to hear about a dog's progress. Also, your experience might help others in the same situation.