Hi H.,
Any medical professional will tell you that toileting during the day and night are completely different. I have three kids - my oldest (boy), 7-1/2 just stopped needing pull-ups about 3 months ago. He still has an occasional accident and we take him to the bathroom around 11 pm before we go to sleep. He has absorbent liners under his sheets. My 6 year old (boy) started staying dry through the night at around 5 years and has always gone the whole night through without needing to be awakened. My 3-1/2 year old (girl) started staying dry through the night about 3 months ago. Like my oldest, she still has an occasional accident and we take her to the bathroom before we go to bed. There is plenty of information online about toileting overnight. With my oldest, we tried limiting liquids, taking him to the bathroom a couple times a night, taking away the pull-ups so that he would feel the wetness, etc... In most cases, the body has to mature into itself with regards to bladder size, control, and awareness.
Starting with naps is a good move - be sure he goes before his nap and work on that as a first step. Talk with your pediatrician or read all the wonderful resources online!
Don't worry about it!
W.