We had this problem with my little boy when he was that age. It was for a few months, it was in the morning and after a nap. We still had a monitor in the room at that time so as soon as i would hear him i would run in there and tell him its ok, we are right here. Besides probably being a phase he was going through. I kinda just tried to get his mind on something else, not ignoring his crying, but not giving him all kinds of attention for it. Bring up something like, asking him if he is hungry for his favorite food, or if he wants some juice, or if he is ready to go watch his favorite movie or show, or if is ready to go play outside with his favorite toy. Also, just be really happy to see him and give him a big hug. Thats what worked for me. Just don't really acknowledge his fake crying.
A question to ask yourself too, is how did he fall asleep? Was he crying? Because i always notice if he goes to bed mad or sad, thats how he wakes up. So i always give him a warm bath and read a book before bedtime. This tends to calm him down and relax. If he needs to calm down for a nap, i just read him a book or two.
You are a working mother, so does he go to daycare? If so, have you checked with them, how they do naptime and what time of day?
Hope i have helped somehow. Good luck, but remember, he won't do this forever, eventually he has to grow up!! :)