My first thought is that Dexter is perhaps feeling unwell or is in pain of some sort. Dogs can be very good at hiding pain, but grumpiness and not wanting to be handled can be indications of pain. They can also be indications of fear and anxiety. Has anything changed within the family or house that could be making Dexter worried or fearful?
A growl is a warning and should be taken seriously and respected. So for everyone's safety I think you should make a house rule for now that nobody goes to stroke to Dexter until you discover the reason and can start putting things right. When he's growling Dexter is giving out warnings that he's unhappy about something and if the warnings are ignored there may come a time when he just goes straight to biting instead. I'm not trying to alarm you, but that could happen if people ignore the growling.
If he was mine I'd probably take him to the vet for a thorough check in case the cause is physical. If nothing showed up I'd then look for a good behaviourist, one who understands positive training and isn't a Cesar Milan fan. Sometimes a fresh pair of eyes, particularly those of an experienced dog behaviourist, can see subtle things that you as a family might not have noticed in your everyday interactions with Dexter.
Please do not worry about Cocker rage. It exists, but is very, very, very rare. The name has been used in the past as a kind of general label for behavioural problems that haven't been diagnosed or understood far more than it has for the real thing.
I hope you find some answers very soon and in the meantime please stay with us on the forum. If you need a behaviourist someone on here may be able to recommend a good one in your locality, and there's always plenty of support on here.