Hi Jennifer -
Sounds like you have mostly ruled out any physical discomforts that your baby may have had to contribute to the nightwaking. Temperature in his room could be part of the issue, but I suspect that's probably not the main cause of his waking. If you think temperature is a significant problem, dress him in P.J.'s with feet to keep him warmer, but don't over-heat the room. But, again, I don't think this is the main cause of his waking.
What your son is most likely going through is the very normal, very necessary stage of separation anxiety (i.e. missing you). It's all part of the wonderful way that we humans develop our sense of awareness and it's just another step in their development of trust in you. Unfortunately, many parents have been led to believe that once a child sleeps "through the night" they will/should always sleep through the night with very few exceptions. Having now raised 4 babies with a 5th on the way, I can say with confidence that this is a sleep *myth*, and a very unhealthy one at that. It sets parents up for stress and frustration when their child starts waking up after sleeping through the night for some period of time (I know, because I did this with my first baby). The *truth* is that babies all go through phases of sleeping soundly all night and then enter phases here and there of waking up either due to illness, developmental milestones, anxieties, teething, etc.
Knowing this and wanting both my need for rest and my babies' need for closeness met in a peaceful way, the best solution I have found is to have my babies sleep with me. This way, no matter what the sleep phase, I'm always there to help my babies sleep if they start to wake, and getting them back to sleep when they do wake is much quicker and peaceful when I don't have to get out of bed and they realize I'm right there with them before they get all worked up crying hard. I just roll over and nurse them back to sleep as I drift back to sleep as well. My sleep cycles are hardly interrupted and with a little practice, it's really so easy I hardly even remember waking up when we do wake for a few seconds.
Then, as my kids get into the older toddler ages, I gradually wean them from my bed and never have any significant problems doing so as long as I am gentle and reasonable about the time that it takes to allow the child to adjust. There are no agonizing nights of making my child "cry it out", nor do I ever have to worry that they'll never learn to sleep in their own beds if I don't make them sleep in their own rooms/beds during infancy. Three of my four kids now sleep in their own beds at night and they all had their turns of sleeping with my husband and I. Speaking of my husband, he loves cosleeping because he gets a good night's sleep too!
Some parents are leary of having their babies sleep with them, but it is truly a blessing for many parents and children as long as it is done safely. http://askdrsears.com/html/7/T070600.asp
If you decide to try it, remember that with most new routines you try with your baby, you should be prepared to try it for at least two weeks before you decide if it's going to work for you or not since it may take some adjustment and time to get comfortable. I highly recommend Dr. Sears book called "The Baby Sleep Book" as well as his book "Nighttime Parenting" for info on sleep options and recommendations for safe cosleeping. Another book that is helpful is called "The No-Cry Sleep Solution" by Elizabeth Pantly.
I hope this gives you some other ideas to help you and your baby during this time of change...
Blessings,
J.