If you are thinking about changing to raw, have a read of this:
http://www.cockersonline.co.uk/discuss/index.php?topic=97551.0A lot of processed foods are full of additives and cereals which are of no use to dogs and just bulk up the food with carbs. There are decent kibbles, but, personally I am a big fan of raw and so is my dog

Don't get too hung up on weight charts - cockers vary in size and build and I have known some little ones at 8kg and some big ones at 18kg. You can tell if your dog is overweight by eye. You should be able to feel his ribs easily but not see them and from above he should have a clearly defined waist. When he meets these criteria, then weigh him and that is his ideal weight

With food, you can adjust the amount you feed accordingly - some dogs are prone to putting on weight and some burn it all off. Not sure that being neutered is really a factor - the main factors (as with us!) is amount of food and level of exercise.
I have fed Coco raw since she was 6 months old (she is 12 this year) and it really suits her well. I keep a close eye on her weight (anything around 12kg is ideal for her) and adjust her food accordingly. She is a very greedy dog and ALWAYS looking for food, but I bulk up her meals with pulped veg, a raw carrot every day to stop her putting on weight.
Plenty of complete raw foods around now - easy ones to get hold of which are both excellent quality are Natural Instinct and Nutriment - I use both, together with some DIY (chicken wings, carcasses, rabbits and offal). Depends on what suits you and your budget really.