His resignation is a verbal information only and if he completes probation in between confirmation letter would be in order if things are fine otherwise about his overall suitability.
Notice period after confirmation would be different and resignation can be processed as per rules if and when and if he actually gives written notice of resignation.
Tendering /Submitting Resignation Application/Letter on and after Confirmation Letter has been issued/put through is a situation, separate from the Resignation Application/Letter in course of and during the Ongoing Probation Period altogther. After Confirmation, Notice Period must be Given/Observed whereas Resignation during Probation is without Any Notice Period.
Be guided , accordingly.