The short answer is yes, a man with only one testicle still has the chance to father a child.
Men who have two fully functioning testicles produce sperm and semen in both. If a man is born with only one testicle, or has one removed later in life, the other testicle will resume normal function and produce sperm and semen for both. In fact, fertility is said to be "unaffected" by the absence of one testicle. Given that in a single sperm sample of a healthy man, there can contain an average of twenty-million sperm, even if that number is reduced by half, the potential for fertilization is still very high.
The presence of seminoma, however, indicates that this man has (had) testicular cancer. At 25, an average man would not discover testicular cancer unlesshe was already having fertility issues. So while the possibility of fertilizing an egg is still high in the case of having only one testicle, the presence of testicular cancer might mean that this man already has a reduction in "normal" fertility. Again, there is still the possibility to "father a [natural] child," whether that comes naturally, using fertility treatments to increase the odds, or going as far as using artificial insemination.