Kings Brompton is in the Exmoor National Park, roughly in the middle of a square drawn between Minehead in the north, Taunton in the East, Tiverton in the south and Barnstaple in the west. It was previously known as Brompton Regis.
New records are obviously coming online all the time and, according to Ancestry, John has now been allocated the same father as four other Musgroves born around the same time - William.
Without visiting the Taunton Record Office I can't substantiate either alternative. They may also have bastardy records which might provide more details of the father.
Taking the results from the Ancestry records, I've now tried to find John's alleged siblings on the 1841 and subsequent census to see if any connections can be found which might prove the working theory that they all had the same parents. Sadly, I've drawn a blank. Perhaps they died ? Perhaps they emigrated ? It is all very frustrating.
In September 1865 Robert and his son, John (born about 1842), came to blows as described in the attached article from the North Devon Journal.
Doesn't sound like one of my family !