R.F.
Please read "healthy Sleep Habits, Happy Child" by Marc Weissbluth, MD. His intense sleep studies give great insight into the development and sleep cycles of children from infancy to adolescence.
5mo babies need sleep and simply don't "decide" they don't need naps (barring a medical problem.) He has a window of opportunity wherein you can soothe him to a drowsy state and he will sleep well. Beyond that, he is "overtired" and cannot maintain a sound sleep. You would need to give a lot more detail in order to elicit more helpful answers, but I strongly suggest Weissbluth's book.
Good luck