Um, babies change so much....each developmental change they go through causes a hiccup in their "usual" sleep habits. It could be too that maybe your baby is going through "separation anxiety" and/or physical changes ie: the ability to roll over, sit up, stand, cognitive changes etc.
(oh another thought...maybe, you are putting him down for his 2nd nap too late? The time span between his first nap, and his second nap is sorta spread out. Maybe by this time he is just too "over-tired." Over-tired babies/children actually do NOT sleep better and actually have a harder time falling asleep).
Babies have a pattern of: wake, feed, awake time, nap/sleep. The window of "awake time" is about 2-3 hours... then they are tired again. At this age, even a bath and diaper change is an "activity." No need to get them "too" wired-up and active all day just to get them to tire them out and sleep later. Sometimes, this back-fires.
(1) How? Consistency, and routine. Keep it consistent... everyday. Same routine, same time, same sequence, same nap/sleep place, same everyday. *Also, let the baby wind-down before nap or sleep. Don't just plop them down after playing or something exciting. They NEED to unwind, first... pre-nap preparations.
(2) For me, since my son for example was a baby (he's just made 24 months old)... he napped twice a day, and still does. He wakes in the morning about 6:30a.m. and has his morning nap at 9:30 (I put him down at this time, and even if still awake will fall asleep by himself). He naps for 2 hours. Next, he wakes, has awake time, eats etc. His next nap is at about 2:30-3:00... he naps for 2 hours. My 5 year old daughter ALSO naps at this time, the same time as my son, and they both nap for 2 hours. This is their rhythm... and the routine I have done consistently since they were babies. Oh, as babies, I would nurse my kids before each nap, and after each nap, before solids, before bed, and during the night IF they were hungry.
3) if my baby/child does not sleep...for my son- I go in, check his diaper (he will not sleep if he has poop) I don't talk to him or interact, put him back down, say "sleep..." and walk out. He doesn't cry... and will go back down. IF he still does not fall asleep (on the rare occasion during growth spurts or developmental changes), then after say 45 minutes, I let him out. At night, still... they go to bed at the same time, regardless if they nap or not. Even with their afternoon nap, they still sleep fine at night. My girl goes to bed at 8:30, my son at 8:00. Due to their ages.
If my eldest child does not nap (on the rare occasion), I tell her that it is "quiet time" and no talking or waking up brother and Mommy is resting too... because she is older, this is fine with her and she usually does comply.
My kids now.. after being used to this routine and habit... I just tell them... "okay, nap time in 15 minutes...get ready..." and they do! I'll change my son's diaper, he has some milk first, maybe read for 10 minutes or watch his favorite "Little Bear" video for 15 minutes...this makes him un-wind... then off he goes. My son has even on occasion just walked downstairs on his own, grabbed his favorite stuffed cow, and walked in the bedroom and climb into his crib. My girl, will put on her pajamas...go pee, and go into her bed. They both sorta "hang out" at first in their crib/bed... but then they fall asleep. No, it's not "magic"... I worked hard on this and consistently. It took effort. Now, they don't even "protest" about nap or sleep time. Rarely.
This is just my routine I worked out for myself, trial and error, but it WORKS! I have always had a "nap" routine as part of their day....it was not nilly-willy or erratic. So, they are "used to it." THIS is the key, I have found. They nap only at home... this is where they nap best. I do not take them out or on the road at their nap times. I keep it regular. PLUS for me, because I have a "schedule"... it makes it easy for me to plan my day/activities/appointments etc. because it is "predictable."
Well, have to get my kids down for their afternoon nap now... hope this helps. Again, consistency, regularity, and routine. The same everyday.
Good luck!
Susan