My DD used to be a catnapper also, we really didn't get some sort of routine til 7 months. Our feed/ play/ sleep was everywhere, I just went with the flow. If she wanted a feed she got it and I put her down when she was tired. Her play times used to vary quite alot.
Also too keeping him up longer won't necessarilly make him sleep longer, if he gets too overtired he could be harder to settle.