
The red brown colour on the black coat is perfectly normal and it appears more obvious as the coat dies and the dog sheds his coat.
As to the 'curly patch' on his rump it is a lot more common than you might think looking as pictures! The breed standard only requires the hair to lay as flat as possible and I know several who have a real swell of wavy hair over the rump. I have also seen one which was very curly haired all over his body. some dogs have unruly hair and as they mature it seems to straighten out more.
The curls can be tamed, and many show folk will use a special coat which they put on the dog immediately after bathing to encourage the hair to lay flat.
Even if he is too curly coated to show, there are still many other options open to you as a flatcoat owner. They really enjoy using their brains. (Yes they have one not all are scatty). Many flatcoats which are shown are also worked in the shooting field but if that is not your choice of activity there are a number competing in agility and obedience these days.
Are you a member of the Flatcoated Retriever Society? If not and if you really want to do something interesting with your dog I would recommend you join the Society, we have experienced FCR folk who already compete in obedience, agility, tracking and shooting. Showing is not the only discipline you can follow with a flatcoat, which is really very useful as there is nothing so pleasurable as watching a dog work, using its mind and thoroughly enjoying itself.
http://www.flatcoated-retriever-society.org/