K.,
This is totally not surprising. Many kids start to experience separation anxiety at about 8 months, and your son is just the right age. It is a phase, but one that can last for a really long time. My son did the same thing. Slept through the night from 2-8 months, and then started night waking. He's 2 1/2 now, and almost never wakes up at night.
My advice is to begin a consistent sleep regimen with him now. Bath, pajamas, story, hugs & kisses, and then into his crib. If he is falling asleep in your arms and then being upt down, he never learns to fall asleep, so when he wakes up away from you in the middle of the night, he freaks out, and doesn't know what to do to help himself go back to sleep. Mike responded very well to the Fisher Price aquarium soother, because the lights and music entertained him, and then lulled him to sleep. He would wake up, hit the button a time or two, and fall back off watching the little fishies move.
It will end. I promise. I can't tell you how long it will take, but if it's gone on for a month, it's not his teeth. Everyone has their own feelings about this, but I'm not an advocate of Cry it out. If he cries, respond. Tell him a story, rub his back or belly and sing him a song till he's ready to sleep, and eventually he will know that you are only a yell away, and it won't be so scary to be away from you. If you don't respond, then all he learns is that he really is on his own in there, and if he's upset or afraid, no one's going to help him. Talk about frightening. He may not bother to cry for you in the middle of the night anymore, but you'll probably be setting yourself up for big-time bedtime battles. If he knows he is truly alone after bedtime, he won't want to go.
Best of luck, K., I've been there, and my heart goes out to you. If you are a stay at home Mom, try to sleep during his nap. Every little bit helps!