I follow Sayed Muhammad Hussein Fadlallah as a marja and my husband follows Sistani and he would like to change his marja to Fadlallah but he apparently can’t because he has to follow an Ayatollah who is still alive but because I am his wife can he still switch to Sayed Muhammad Hussein Fadlallah as his marja?

No. He cann not, but you can switch to S. Sistani for sure.