Fee, I wouldn't worry about it too much. It would be nice if we could always follow that routine, sleep/feed/play, but unfortunately like Mads said, they change often, and he could be going through another development stage.
In 'The Wonder Weeks' book, they say another development stage is at week 19, and I think your DS is 18 weeks old, so it could be this?
In the mornings, if he isn't intersted in feeding or playing, put him down in his bed or somewhere and if he nods off or is just happy to lay there and play, then that's ok. If he doesn't want a feed, play or sleep, then maybe take him for a walk or have cuddles with him. He may just want some mummy time?
I know it's hard when you think you have them figured out and they change it on you. Good luck with it all.