Having ONE testicle needs to be confirmed because once in a while the other testes is UNDESCENDED meaning it may be in the abdomen and so may not be visible outside.For an undescended Testes
the main problem is NOT on testosterone level
but of finding out and removing the hidden testes to avoid getting a testicular cancer
in that testes so if you have not undergone ultrasound
for the same you may do so or meet urosurgeon and do the needful because it is best to locate and bring down or remove the hidden undescended testes.Do get back if needed.The testosterone level in blood can be eaasily checked and in any case there need not be any worry about having children so not to worry.
Best wishes and take care