When could he hypothetically cum in me?

I’m not on birth control. My partner and I were discussing the other methods around preventing children again recently. We’ve been together a little over a year and condoms/pulling out has worked well for us.

He had asked me if there was a chance he could cum in me once and then I take a plan b. I explained to him the dangers of taking plan bs and regardless we would have to plan that out with my cycle. But I’m not very sure when is the better time to do such things? Would it be during or slightly before the period? Or after?

We wouldn’t do such acts if it would require me to take a plan b as those can hurt my health (and I’ve taken one in the last year) but is there a better way to try it with low risk of pregnancy instead? I know either way it would be foolish but we would be fine with a child if things go wrong. We would just prefer not to have one at this time.

(Please no judgment on picking to not be on birth control.)