I found at that age just to put them down for the night when they're tired. We used to have a rough bedtime but would just see what time our DD woke from her last nap, when she was getting tired etc. I think it takes awhile for babies to fall into there own sort of routine.
Also putting babies to bed later don't always guarentee that they're going to sleep longer. Maybe you could try a dreamfeed before you go to bed to get him to stretch out longer? That works with some babies but not all.