Hi C.,
Your son is doing this because he is getting to an age where he now can start to feel separation and absence of objects and people. When babies are first born you can put them in a bouncy seat and walk away without them looking for you or getting upset, but as they get older around 5-6 months they are able to think about you and know you are there, but cry because they can't see you, so that's why Dr.'s say their sleeping patterns change as they age. They do this with toys too. At this stage they will have a toy in their grasp, drop it and start to look for it because they remember they had it in their hands. I know Dr.'s and even friends say let them cry it out, but for me and my husband, we couldn't do that with our daughter. 5 minutes was our max!!! I'm up with her sometimes at 2am for a long stretch, but that's her teething now, she has 4 coming in at once, poor dolly!!! Babies through toddler years just need love and attention and extra hugs to feel secure. They grow so fast and the nurture you put into them now will pay off later!!!!! One suggestion is to get a good white noise machine. We started our daughter with this the day she was home. White noise helps as a soothing background to drown out other noise and really helps to lull them to sleep. It does WONDers for our daughter and she sleeps very well now at 13 months old. I hope this advice helps.... He will start to sleep again, not to worry! Just comfort him when he needs it! =) Sending sleep vibes!! =)