Partner masturbated an hour before we had sex and did not urinate do we need to be concerned about left over semen from his masturbation?

Alive sperms from the last masturbation may still present in his precum even he does not ejaculate this time. You are correct about your concern but I think the chance of conception is low because of the low quantity and poor quality of those sperms. However, to be safe than sorry, you should take plan B if available. Good luck.

Not from left over semen - for the sperms would have doed within 10 mits of ejaculation- but the time you had sex