It's possible, but finasteride doesn't cause weight gain or muscle loss. In fact, it slightly increases free testosterone due to less of it being converted to DHT. He probably just got older and became less active.
Yes, finasteride can cause a very slight estrogen increase (~15%) but nothing like bodybuilders who inject anabolic steroids for example (3x-5x+ increase). It's similar to the estrogen bump from gaining 10-15 pounds of fat. In any case, it doesn't cause weight gain or muscle loss.