I agree with most of the others that you should just put him back in pull-ups for now and don't make a big deal out of it. Nighttime potty training is not the same as daytime. It's more related to how their bladders work and how deeply they sleep.
My older daughter wet the bed every night until she was almost 4 years old. Then, she told me that she didn't want to wear pull-ups to bed anymore, so we let her try just undies. I'd say that for several months, she still wet the bed at least 3-4 times per week. I did buy a mattress cover (goes over the sheet so that you don't have to change the whole bed), and that helped make me feel better. We tried not to make a big deal out of it, and she did eventually outgrow it. She's now 5 years old, and we have no problems with bedwetting.
My younger daughter day trained herself by age 2, and she started wearing undies to bed at age 2 1/2 (the same time as her sister -- they're 17 months apart in age). She actually wet the bed less often (although still once a week or so for a little while). She's now almost 4, and I can't remember the last time she wet the bed.
Kids are just so different, and my guess is that your boy isn't ready.