I have a Cavalier who is now 18 and doddering. I wish he could jump but even when he was young, he was very grave and serious. As a long-time dog owner (I currently have 5) I have learned to add a hand signal with a the command. For instance, I always use my index finger almost like a scolding librarian, without the scold when I say Sit!
Keep your energy very level and calm. Your Cav will learn quickly. You might also look into clicker training.
Our Basset is verrry stubborn and will do what he wants to do unless I click that clicker and BOY! he comes running...there's a treat there! I really do like clicker training. The main thing is consistency...without that, you're lost.