@surftohealth88
Mrs. Surf-Just another data point for you both as your husband considers switching to E2: I had some rough days (strong fatigue, slight nausea, strange head pressure) at the start of taking E2 (6/17) , but I was also taking Zytiga/prednisone and getting RT.
I am happy to say that the symptoms decreased over time and that for the last 5 days I have felt really good--all those symptoms are basically gone! I was able to bike over 20 miles (over 1000 feet elevation gain) today for the first time since starting treatment. I think you were right that my body just needed to get used to the big hormonal changes (T went from 350 to <3 in a month). Since your husband has already experienced his major change in hormone levels from the drug he is taking, perhaps he won't have any of these symptoms.
In any event, I hope he continues to feel good and get good results!