Al Sheikh Dr Hasan Isa Abd Al-Dhahir says: 

It is best to refer to the medical examinations and analysis to find out the health of the testicles, and whether it can do its function. If it appears fine and performs its function in a way that does not affect or hinder the reproductive process, then there is no need to inform the fiancée of that. However, if the results of the examination and analysis confirm the loss of the second testicle and has an effect on the sperm and will be an obstacle to childbearing in the future, then it is necessary to inform the fiancée and allow her to choose between accepting or not. 

An infertile man came to Umar bin Khattab and told him he was engaged to a woman and he had not told her that he was infertile, then Umar ordered him to inform her and allow her to choose.