Will he benefit from another dog?

I have a dog (Charlie) who is 7 years old. He's always been a pretty excitable dog but we were hoping he would calm down a little as he got older, but that hasn't happened. We try to calm him down a lot but he just barks and runs around, driving my mother insane.

We were thinking that, when the time is right, we would get another dog (another CKCS) but will that calm him down or only serve to get him more excitable or hyped up?

Probably you are not meeting Charlie's energy needs. A few things to consider would be a training class which could work on helping him to stop barking, and get rid of excess energy, doggy daycare would let you get an idea for how well he plays with other dogs and wear him out, fetch is a really easy way to exercise your dog without much input from you.
If you get another dog it is possible they will play and wear each other out, but you could also end up with 2 barking dogs. Puppies tend to pick up good and bad habits from older dogs, so you'll want to get Charlie's barking under control before adding a new puppy.

It sounds like he needs things to do. He is probably bored. I have got three dogs and two of them are very happy with their walk and then sleep the day away on the sofa. I thought all Cavaliers are the same but when I got Harley I was proven wrong. I take him to flyball, agility and obedience every week and after he gets home he sleeps for an hour and then he wants to play fetch.

I had the opposite, Barney is a very chilled out eight year old Cav, he slept a lot in the day until we got Cassie ( now 13 weeks) It took him a week or two to be fine with her as he's been our only dog but now they're great together...She peps him up and he's playing more and they chase each other a lot...On the other hand Cassie picks up on Barneys chilled out character and good manners and is learning lots from him Best of luck if you get another!!