S.M.
Hi, E.! This could be separation anxiety, which is common at this age; he will eventually grow out of it, but if he develops a habit of sleeping with you all the time, this will be a big problem. He could be a little ill from teething, too, and need some comfort. If he can't stay asleep after he falls asleep in your arms, I'd say there is something physical that is troubling him; teething is the first thing I would suspect. Time to break out the Orajel and the baby Tylenol. Try that first; Tylenol about 20 mins before bedtime, and the Orajel as you're rocking him to sleep. Then see if he can't be gently laid in his bed, at least for a couple of hours. He may be getting molars at this age, and those are BRUTAL to his poor, little gums.
If it's not physical, then he could be having the beginnings of nightmares, but he can't tell you about it. Try using a nightlight. There are wonderful softly lighted displays with a little quiet music that help. Lavendar-scented baby wash before bedtime helps, too.
If it doesn't clear up, I'd take him to the Pedi.
Hope everybody will be fine,
Peace,
Syl