Ttc. does anyone know the signs that could mean that a guy cannot father a child?

Several reasons: The best determination for ability to father a child would be to start with sperm evaluation for number produced and motility (movement). If a male has only 1 testicle the sperm count is lower. If a male has undergone chemotherapy, the sperm count may be lower. If a male has a chromosomal abnormalilty XXY klinefelters syndrome, sperm count may be lower.