Here's a few things that could help:
-making your room really dark. We tacked up blankets on our window when ds was 5 months and it really made a difference. When we were out, he could still fall asleep if it wasn't dark. You don't have to worry about him learning to only sleep in the dark. Darkness helps cue the body it is time for sleep, the body will actually produce hormones that help you sleep.
-white noise. I have never used this since ds is pretty good at sleeping though sounds. But I've heard tons of great things about them from people that have used them. An example of white noise is the sound made by a fan or air condition, that gentle humming sound. It's helpful because 1) the gentle, consistent sound can be effective at soothing a child to sleep, 2) it can filter out other noises that may jar him awake (like the phone ringing, a dark barking, ect), and 3) it creates a cue for sleep. You can use a white noise machine, white noise CD (like ocean sounds, waterfalls, ect.), a house hold object (like a fan, radio turned between stations, ect.)
-having a bedtime routine. This doesn't have to be dependant on times, just have the same order of activities. Example, read a book, sing a song, swaddle, nurse to sleep. This helps to cue sleep.
-leaving a shirt of yours (that has been worn) behind with him so that he can smell you and sense your presense.
About the sensing that you are no longer there. I think they just grow out of this as they age (if that's why he is waking). Ds would often wake up shortly after I left, and I would go back in and nurse him back to sleep. And the older he got, the better he became at sleeping on his own. Just try to make sure he is in a deep sleep before you leave the room.
In terms of when he goes to sleep. I don't have have much advice other than to say that it will get better with time. I strongly reccommend reading these two books: The No Cry Sleep Solution by Elizabeth Pantley and Sleepless in America by Marry Sheedy Kurcinka. They should be in your local library. They will both have a lot more ideas for you.
Sleep has been one of the biggest frustrations in my parenting journey as ds has never been a perfect sleeper. But the older he gets, the better of a sleeper he becomes. So on days where you are feeling stressed out over your baby's sleep habits, just now that they will get better on their own over time. You can do things now that might really help, but if you keep doing things the way you are now, they will eventually get better on their own. Don't feel that you are doing any damage by not fixing the situation. That was one of my fears when ds was a baby- that I was setting ds up for a life time of poor sleep habits because he wasn't sleeping at regular hours, couldn't stay asleep very long without needing me to nurse him back to sleep, because he was nursing to sleep to begin with, ect. Your ds will eventually be independent and not need you to put him to sleep and he will eventually sleep through the night! Don't let anyone worry you into thinking it won't happen unless you do such and such (like cry it out, which is sadly very popular in our society).
Sorry this is so long, I just know what it's like to be struggling with sleep issues. Feel free to ask anymore questions you may have. I hope that some of this helped!