I have a one month old daughter who still hasn't completley settled into a sleeping routine although she sleeps for as long as she needs to which cab anywhere from one hour catnaps to 5 - 6 hour long marathons!
Some things that we do are:
Do you sleep him in your rooom?
Sometimes babies like to be able to hear mum and dad sleeping when they wake up and it can settle them enough to go back to sleep.
Do you sleep him with you?
I find that with Talana, if i lay her in bed with us after her morning feed she will sleep anywhere from 4 - 6 hours and it gives a chance for a bit of sleep and a cuddle!
Do you give him a dummy?
Whenever Talana is overtired we give her a dummy to settle her and it puts her to sleep.
Maybe your son only needs to sleep for three hours at a time?
Hope these things help (if you havent tried them already!)
Mummy to Talana Michelle Kimberley (13/12/2003)