Nothing.
Discuss in detail his fear(s), suggest he choose the condom and open it.
Advise him your not on your period and several days either way it normal.
Open, honest and constantly communication is the answer.
I recently asked my boyfriend to stop having sex: my reason was because even though we practice VERY safe sex he always panics about silly things such as if there was a leak in the condom and even if my period is late! what could happen if i stop sex?