I'm so sorry to hear what is happening.
As Karlin said a Grade 3 heart murmur, in a puppy of 9 months, needs to be assessed by a cardiologist.
I don't know under what circumstances you obtained Charlie, but if it was through a rescue organisation then they have a responsibility to you and to him. You need to ask them to pay the referral fee for Charlie to see a Cardiologist.
I find it hard to believe that he was not vet checked before rehoming, or that they would not be willing to help, if he was placed with you without a basic health check.
Some of the cavalier club rescue organisations have thousands of pounds, and if Charlie came through one of them they should pick up the health bills, at least until you know what is wrong and what the prognosis is.
Many other rescues, I know, struggle to find funds. This does not mean they can cut corners and lumber someone with a young dog that may need expensive specialist attention.
Something seems to have gone very wrong with rehoming procedures here.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi