I had a day catnapper and it was SOOO hard because he really needed more sleep than he was getting. We tried absolutely everything, read heaps about it and I eventually came to the conclusion that some babies are just catnappers until they get older and there's not much the parent can do to help it. My DS grew out of it about 10-12 months from memory.
I'm sorry if that's not the help you were after but I truly believe they just need to grow out of it. I hope accepting this will make me less stressed about the situation if my next bub is similar.