My brother lost a testicle at very young age to torsion. he is 30 now and had normal development. can he father children and have normal fertility.

All he needs is one!: Our bodies are quite redundant. In other words, we can get by w/one eye, one ear, one lung, one kidney, one testicle, etc (not that we'd want to do so). Since you reported he's developed normally, that suggests that his remaining testis is producing enough testosterone and presumably sperm too (very rare to have one w/o other). So he better wear a condom unless he wants children (and/or stds).

Yes: Most people with torsion have normal fertility there are some theoretical questions regarding torsion leading to antisperm antibodies but he should be able to father children.