Doctor to evaluate. A ten month-old baby who starts crying hourly at night has a problem and should see his doctor. The problem could be ear pain, throat ulcers, head ache, tummy pain, tight clothes, fever, or something else that's preventing him from staying asleep. He may be starting a new viral or bacterial illness. If he is much better in the morning, that is reassuring, but his doctor needs to evaluate him.
Many reasons. He may be hungry, or coming down with an illness, or just experiencing separation anxiety. Try feeding more solid foods or giving breast milk or formula at 11 pm. He also may be teething, so a dose of tylenol (acetaminophen) at bedtime could help. Try to teach him to fall asleep on his own in the crib instead of falling asleep in your arms so he knows how to fall asleep again on his own when he wakes.