Dr. Greene's Answer
Most young kids will return to sleep well if rocked, or cuddled, or fallen asleep with (of course, whether or not you want to do this is another issue).
Sometimes adjustments in bedtime or napping can make a difference.
But when kids don’t sleep well in the most conducive setting, I like to have them seen at a sleep center. Thankfully sleep is beginning to get the attention it deserves, and sleep centers are springing up in many places.
Occasionally sleep specialists recommend medicines after other options have been explored. I am in favor of this (though I am not a fan of the benadryl class, at least as either a medium or long term solution). Sleep is so valuable for health (his and yours) that in some situations the benefits clearly outweigh the costs.