R.H.
I know it sounds weird but maybe he's overtired. I know around 5 months babies really start to develop their natural sleep cycle. If he isn't getting his proper naps and as much sleep as he needs he may be overtired and then they have a hard time sleeping at night. He should have his first nap 2 hours after he wakes up in the morning. Depending on your soothing routine I would start soothing him about 1.5 hours after waking up in the morning ie breast/bottle, story, lullaby, dim lights etc. After he wake from first nap next nap should be about 2 hours later and the 3rd nap about 2 hours after he wakes from the 2nd nap and bedtime would be 2 hours after that. For more insight on sleeping schedules read Healthy Sleep Habits, Happy Child by Dr. Marc Weisbluth. When my daughter was 5 months old she started getting really fussy and I realized she wasn't sleeping enough. After I read the book and got her on a good schedule she's been a champion sleeper ever since. She is 18 months old now, still in bed by 7pm at the latest and sleeps until 7 or 8 in the morning. Everyone comments about how good natured she is and I think it's because she's well rested, and so is the rest of the family! Good luck with your little guy.