Don't rule out the possibility of something stressful or traumatic going on that's the cause. My older brother wet the bed till he was at least that old, maybe longer. He had a pretty stressful life growing up. We all did. Our Mother was a rageaholic (among other things). We never knew from one minute to the next what was going to happen while dad was at work. Lots of therapy later, we're much better. Hold off on the liquids after 6 pm.
To help your 8 year old boy from wetting the bed, you can try bladder stretching exercises. It is advisable to first talk with your pediatrician to see whether your child should do these exercises. Alternatively, you could try to monitor what time of the night he tends to wet the bed then set an alarm to wake him up like ten minutes beforehand.