Sleep is a learned behaviour. Adults go through the ritual of getting ready for bed, turing out the light etc. We have several awake periods each night and we quickly assess our surroundings. If we went to bed with the light on and it is now off, we will awake more fully. If our pillow is on the floor, we re-adjust it and of course awaken.  Children do the same thing. If they were rocked to sleep or went to sleep with a bottle or pacifier in their mouth or with a light on, that is the way they have learned to go to sleep. Put the child to sleep in the same conditions they will find their room when they awaken. If the child wakes up and won't go back to sleep, quietly go into the room to reassure them that you are still there, and gently tough them on the back. Don't talk to them or encourage their behaviour in any way. You are there to alleviate their fear that they are not alone. Stay with them for 5 mins the first night, 4 mins the next etc. But dont talk to them, give them a drink or re-adjust anything, otherwise you will be reinforcing their behaviour. This process takes time, patience is a virtue.

Good Luck! 

In addition to the some of the other suggestions, does your child have any skills in comforting himself?  I'm thinking of things like the favorite soft blanket, doll, thumb, etc.  If not, maybe he should be encouraged to have an attachment to something that can take your place when you're not there.  My son used to be extremely attached to a cloth diaper, which he even talked to.  Of course, then you wonder if you'll ever get him to give up the attachment, which is what I feared, because I tried just taking it away and he was devastated, even though he had agreed to it.  What we ended up doing was putting it inside a small pajama bag puppy which went under his pillow and he never asked for it again.  I think a lot of babies have trouble transitioning to sleep; I know my 3-month-old grandson uses his pacifier for this purpose.  If you still have trouble, I would suggest talking to his doctor about it - maybe there is something going on physically that is not obvious.  Hope this is helpful.