Their personality will not change at all from how he is right now
. If anything, by neutering, he will *not* gain some aspects of personality many people really do not like! He will change more by not neutering than by neutering. Also, by neutering you greatly reduce the risk of him wanting to wander (a male scenting a female in heat will often do anything to escape -- and will run up to several miles to find her), and other health problems males can have. And you get a more settled personality. If you have concerns, talk to your vet's office -- that is what they are there for.
I've neutered about 30 cavaliers and never had a problem. The boys is an especially fast and relatively simple procedure. And many neuter the boys because some *become* problems when left intact.
An intact boy needs a lot more managing -- around females, around other males (he will generally be more likely to be aggressive), in any home he visits (he will almost definitely lift his leg and spritz urine all over furniture, curtains etc unless restrained at all times or trained not to do this), will likely have increased humping behaviour on people, things and other dogs. Their urine is also very strongly scented. You must keep him on the lead at all times to not risk having him take off after a female in heat, to be lost permanently/stolen. And they are at higher risk of prostrate problems as they age. I've had to neuter intact males of 7 years+ *solely* because they are already developing prostrate problems by that age for having been left intact. These are some of the reasons many people (and vets) prefer to neuter.
Also: whatever about the above -- good breeders almost always have a spay/neuter clause in your homing contract. If you homed from a responsible breeder you likely have an AKC or CKCSC dog on limited registration -- and a neuter clause written in to your homing contract. This means you must neuter.
You can ring your breeder to confirm but generally responsible breeders only sell a puppy in the US on open registration to experienced show people/breeders because of the risk the dog will be used for breeding when s/he is pet, not breeding quality.
NB: I am moving this to the Health section as it is in the introductions section at the moment, where it doesn;t really belong.