Thanks for writing in.
First of all you must get your undescended testis
in the abdomen removed surgically. This is because there is a very high chance for cancer formation in undescended testes
. Please have yourself examined by a surgeon.
Secondly, you can become a father if you have only one testis. However, the sperms produced by you must be normal in number, morphology and motility.
Hope this helps.