Hi Hayely,
Sounds like your lo may be going through some separation anxiety. I think the best way to deal with this is to make sure they feel secure in knowing that you will be there for them. During the day, you could play return games, such as modified peek-a-boo games so he realizes that when you go, you will return. Right now, at night, my feeling is you need to go to him when he begins really crying at night. Leaving him to cry will increase his anxiety and may make matters worse. When you go to him, see if you can settle him in is cot. Once this phase ends and he becomes assured that you will be there for him, he should start sleeping well again.