It could be many things, but most assuredly something temporary! Is the living situation new because that would mess up a sleep routine? If not, at that age it could be a growth spurt, teething, or separation anxiety. Once you make sure that he is not in pain (teething, hunger, temperature, etc.) there is nothing wrong with letting him cry to sleep. His bouts of crying should get shorter and shorter, and within a day or so he should sleep through the night again. Babies and young children naturally wake up often throughout the night, and if they get used to a sleep aid to fall back asleep, they will cry until they get it. So if you nurse/bottle feed to sleep, be prepared to do that every few hours. Pacifiers, music, white noise, etc. can also be a sleep crutch. The more his environment can stay the same throughout the night, the better he'll sleep. Good luck!