My DS was a mare to get to nap as well.
For us, we had to commit to a bit of a routine.
Pick a good naptime based on when he usually starts yawning or rubbing eyes. At 5 mo maybe it's about 2 hours after we wakes in the morning.
Try to wind him down a minute or two beforehand, avoiding last-minute overstimulation. Take him in his room telling him several times as you go, it's naptime now. Take him into his room and announce everything you do in a quiet voice to him as follows: we're in your room now, we're turning down the lights/closing the curtains, we're putting on your music (Lullaby music playing on repeat - key), have a nice cuddle with Mama, and we're laying down now. Here's your teddy, and your blanket, and your dummy. Shhhhhhh, shhhhh (rubbing belly or touching cheek), "naptime now" (rubbing again), "night night" (give kiss) "mama is here if you need me" walk out the door. If he's thrashing about try holding his legs down a few seconds to help him calm himself down. If he screams after you leave, wait to make sure he's not actually slowly calming down, go in and reassure him, pick him up quickly, put him back down and repeat giving of dummy, blanket, rubbing, etc, and leave again. Might take a few times at first. Have patience.
(Up till 10mo we used a dummy, and only at naptimes. It helps sooth them and can be a godsend. He doesn't need it now, just music and teddy and blanket. In fact if he hears his music now he drops off immediately.)
Once they get used to the nap at the same time each day in the cot, they'll probably drop off at this time automatically while you're out. Invest a week or two at home though to get them well used to falling asleep at the same time each day in the same way.
Good luck!!