Yep, yeast infection. Get the old school 7-day yeast infection cream. I was told to apply it 3 times a day, morning, after nap and before bed.
Here's 2 cents: My daughter kept getting yeast infections last summer without any antibotics to bring it on. After the 3rd one, I realized that it was due to the darn Texas heat & humidity plus wet diapers while she was sleeping at night... made the perfect warm and moist conditions for a yeast infection to thrive.
IF it keeps happening, you might want to dry a different diaper at night. Or see if putting him to bed with only a diaper and no pj's over his bottom half helps decrease moisture. Or change him <gasp!> while he's sleeping.