Taking Beta Blockers: How Can I Reset Exercise Heart Rate Thresholds for Apple Watch?

I bought my husband an Apple Watch 4 on the recommendation of his cardiologist, who was involved in development of the EKG feature with Apple. As a heart failure patient, my husband takes beta blocker medications that prevent his heart rate from going above 90 BPM, even during a strenuous cycling workout on mountain bike trails. Apple Watch uses heart rate thresholds (in conjunction with GPS) to track exercise that counts towards closing the Exercise ring each day, so even though he just completed a one hour outdoor cycling workout and came home drenched with sweat, Apple Watch only credited 21 minutes towards his exercise goal. Super frustrating, especially since one of the primary reasons we bought the Apple Watch was to help him stay motivated and on-track with exercise to manage his heart condition. Is there some way to reset that exercise threshold in Apple Watch 4 for a person who is taking beta blocker medication? If there isn't, please include that in the next update!