how do you keep up stamina during fertile window??

So this fertile window we had a LOT of sex. For a week and a half we've been BDing every day or every other day. The past couple of days, though, my husband hasn't been able to get or keep an erection. I know this is probably just because he's totally wiped out from all the sex up to this point, but how can we make sure we're able to have sex through the entire fertile window without literally exhausting ourselves?  Anybody have any tips or experiencing the same issue??