M.R.
At six months he's only going to be responding favorably to his own internal clock. While you can get him to "approximate" timeframes, you won't be able to fully get him scheduled for a few more months.
Things you can try are to expose him to sunlight. Let him play in a sunny spot in the playpen, with you, or whatever. For some reason, kids adjust their clocks easier if they have some sunlight time. When it's time to sleep, move him to a darkened room.
You could also try more frequent bouts of interaction. At six months, getting him used to his muscles will provide mental and physical stimulation as well as tire him out a little bit. Rub his legs, bounce him gently, tickle his toes, blow on the belly--- these things provide a lot of exercise to a small tot.
Unfortuneately, until he is on a schedule, you have to keep a sense of humor and flexibility. Have fun.