My partner is a regular blood donor and has frequent medical check ups. does this imply that he is stis safe?

Not fully: Blood donors are only screened for conditions transmitted via blood (hiv, hepatitis, etc.). There are other potentials (gonorrhea, chlamydia, etc.) that are not screened for, and potentially could be present. Condoms always best choice.

Not necessarily: If a person is a voluntary blood donor should know how to prevent high risk behavior that may jeopardize his or her health and the blood to be transfused. Blood tests are performed to confirm that the donor has not been exposed to possible transfusion transmitted diseases, but there is always a chance of having a false negative result if the exposure to a disease is recent(days or weeks).