I have always had cocker spaniels so will try to help. I am working on the assumption he is a working cocker spaniel.
Cockers vary hugely in size. So do springers. They are essentially the same dog. Springers were the large ones cockers the small. You could have 2 dogs from the same litter and one would have been a cocker and one a springer.
Now it's not like that, but dogs genes can throw back generations. Lots of lines have springer in somewhere fairly recently. Because one of the dogs isn't know to have been a sprocker. I am guessing there's not generations of verified parentage. Springers can also be huge.
Unfortunately, this results in owners getting dogs that are far bigger than expected. One of my cockers is 7.8kg and tiny. I fostered the litter and am still in touch with the other owners. Her brother is 15kg and massive in comparison.
At 8 months old, 5 miles a day is alot. Cockers and springers do need exercise. But there's a fine line between enough exercise and too much. At 8 months a walk should about 40 mins at most. Though this is on lead. Could be longer if off lead. But it still sounds like alot.
And anbover tired spaniel can be worse behaved than an under exercised spaniel.
Brain training, is just an important as physically exercise. Potentially more important. You need to do training, brain games, scent games. Have their brain working, but again too much can make them crazy. With working spaniels you have a while of trying to walk the fine line of enough, but not too much. It is a fine line, until they are an adult. When they have grown up and are fully trained they could go all day.
Having company isn't really enough, they need interaction.
You also have the teenage phase, where training goes out of the window in some cases and you have to start again.
How's is his recall? How is he on walks. Do you work him on walks. By this I mean let him sniff and explore, but also work his brain. So we have the kids play hide and seek, let the kids run off and call the dog then we call them back, recall them regularly and have the dogs focus on us. Our dogs always have an eye on where we are.
I would usually recommend a trainer that works with Working dogs. But my main concern here is that you simply don't like him.
If you have no bond, everything is going to be harder. Spaniels love their owners and if there's a good bond training is easier. They want to please their owners.
If you don't like the dog and can't provide a house that's suitable for a working dog, it may be better to rehome him. I would recommend Spaniel Assist rescue and rehoming, they will find a home that's suitable for him and provide the Foster family with trainers and behaviourists if required.
If you really feel this isn't working and you can't see it every working. It's best to do this sooner rather than later. It's doesn't sound like a happy home for you or him.