Focus on just the daytime first. Part of it is getting the muscles to become more trained and able to recognize the feeling of pressure. That takes time. After he is doing really well in the day time then think about working on night time.
It will help if you try to have him use the bathroom right before going to bed and not fill him up on liquids in the late evening.
I reccommend using the seat that fits into a standard toilet (The one with handles on each side is best). Then place a small step stool in front of the toilet to help with getting up and down.
This will not only help you with clean up, but help with getting him used to the "big boy potty"
I also reccommend not flushing, until he has had a chance to step down from the potty. If you flush while they are still sitting on it, it can scare them. It will also be fun for him to tell the poopy "Goodbye" and let him help flush (I know - it's weird, but it works)
Try to keep an eye on times he seems to most often have a bowel movement and have him try and sit on the potty during those times. Be sure he has the stool under his feet -- It will help him co-ordinate his muscles better for making a BM.
Hang in there and Good luck -- It will really depend a lot on the individual kid
Use what ever you need to , to keep him dry and comfy during the night -- Night time will be the last thing to get learned when it comes to potty training.
PS -- In the day time, Pull ups will not help! -- Just buy the heavy cotton trainers and a lot of bleach!
I have three girls and my oldest has bowel motility problems -- wow! was that an experience potty training!!