Has he always been this way?
Teach him sign language, for babies. Look it up online.
Also, the emotions, in a child this age and even at 3 years old, are not fully developed yet. So they cannot say their feelings nor know their feelings and they don't even know the names for their feelings yet. Hence they cry.
When he gets older, you need to teach him the names for feelings, and how to say it... it is taught. Not an automatic skill.
Now, I have a friend who's baby always cried. She took her daughter to the Pediatrician. The Doctor looked the baby over very closely. It was found, that a blond hair (barely visible to the eye), was tightly wound around one of her toes. And of course, it was getting tighter and it was cutting into the skin and of course, causing pain.
The Doctor, had a hard time untangling and taking off this hair. But once removed, her baby, was a NEW happy baby. For once.
Babies, cry because they are not happy/hungry/teething/have gas/have pains/are uncomfortable/too hot/too cold etc. For any reason. Yes.
That is the ONLY way, they can, communicate.
Try to figure out your child's cues.
AND... ALSO, when tired or over-tired or over-stimulated... a baby cries, because they need to nap... or they just need quiet and they 'cry' in order to SHUT-OUT whatever is over-stimulating them. Be it noise, or people constantly in their face or talking to them or playing with them. Babies cry... as a signal, that they need something or they need something to, stop.
Also, GET the books:
"What To Expect The Toddler Years." Or the book "Your 1 Year Old" (from Amazon). And "Your 2 Year Old" also from Amazon. It is a whole series of books, for each age of a child and it is very easy to read. I merely describes what a child is like at each age and how they are changing. So you will better understand them.
15 months, is ALSO a growth-spurt period and a time of many changes... cognitively and physically. It is growing-pains, too.
Did you take your baby for his 15 month old check-up???? He will also need one at 18 months or 24 months old.
Is your baby... fed well? Is he drinking whole milk? Does he eat well? This is a growth-spurt time. They get hungrier at these times.
You said he is crying and there's no tears.... SO... is he DEHYDRATED??? If so, this is dangerous. You need to take baby to the ER.
Does he have wet diapers? Pooping normally??
Teething???
All of these things, can make a baby unhappy and uncomfortable, too.