I've had two Borders with Mitral valve insufficiency. The first one, Cass, developed a murmur at 10 years of age. I had an ultrasound done and she was put on enalapril. She had ultrasounds done after that (every 6 months, then yearly) and never had any significant change. She died suddenly of a brain issue within a month of her 13th birthday.
I was very upset when my vet heard a murmur on Ziva at 7 years of age. We went to the cardiologist and both the mitral valve and the tricuspid valve have an insufficiency. Since the heart chambers are of normal size, he said no meds at this time. I was upset at her having a murmur so young but the cardiologist said that 7 is the average age for a murmur due to valvular insufficiency. When the chamber size starts to enlarge the first medication that they put them on now is Pimobendan. It is a newer drug and what I find annoying is that since it is newer there isn't a generic form so it is pricey compared to enalapril.
At some point if the heart disease continues to progress then they add enalapril, and later on lasix if there is fluid. Hopefully, it never comes to that. Ziva is due for her recheck in February. If I didn't listen to her heart and hear the murmur (3/6) I'd never know she has a problem.
Her cardiologist recommends taking a resting breathing rate at least monthly. 30 breaths or less per minute is normal.
Good luck with Ben.