Re: 5 1/2 months and can't get to sleep - help!
I would also check his ears...but if it's not that he could be teething. Does he have any teeth yet? is he chewing on things a lot and drooling (i know a typicle baby thing, but they do it a lot more when teething). He may have an ear infection, especially if he's had a cold recently.
If it is teething and he is fine the rest of the day then about 30min before a nap or bedtime, give him some advil (phamasist told me that advil is better for teething) or tylenol. that should help to take some of the edge off and let him sleep a little better. He could just be going through his attachment phase aslo. so it may just be a habit you are going to have to work through. At around 6 months old babies can become suddenly more attached to mommy and daddy than they were before, it's a normal part of growing up and he will outgrow it.