Here's a link for you on sleep/naps:
http://www.babycenter.com/0_how-much-sleep-does-your-chil...
Next, to me, the baby is still tired... but can't get back to sleep himself. He's waking... but over-tired or still tired.
If you put him to nap, when he is already over-tired, a baby can't sleep well nor fall asleep well nor stay asleep well.
Or if a baby is over-stimulated prior to nap, they can't sleep well or fall asleep well.
There is a window of time, that a baby will be tired and need a nap.. if that window of time passes, it is harder to put a baby to nap and they can't fall asleep/stay asleep well.
A baby, typically gets tired after about 2-3 hours of 'awake' time. Look for his sleepy cues.... feed him before nap...
Also, a baby can't always self-soothe at this age, thus with normal intermittent wakings, they may wake then cry... because they are agitated and still tired....
You can try and soothe/comfort him back to sleep.... especially if he is still tired.
Many babies, wake crying... because the are still tired and can't at that age, self-regulate.... or they even startle themselves awake...
Babies also can have a 'pattern' of wakings. Observe him and get to know his cues.
My son for example, at that age, would nap... but after about 1 hour, he'd wake/fuss/cry... but I KNEW him and knew he was still tired. He needed to 'see' me as well... (separation anxiety), and so at that juncture of waking... I would go in, let him see me, and then he'd go back to sleep. He was STILL tired. But, if I interrupted his 'pattern' of waking... and picked him up and assumed he was done napping... well he'd get grumpy and was not rested enough yet, at that point. So I had to 'learn' about his patterns too...
I also had in his crib, baby safe stuffed toys, which he'd amuse himself with... and he'd go back to sleep. Then he'd wake, after a FULL nap (which for him was 2 hours), and he'd wake nice and in a good mood.
just some ideas, all the best,
Susan