There's a chance he's teething, which means his mouth is sore and the paci helps, but he hasn't really made the connection yet.
Couple of suggestions:
1) teach him to grab and put in his paci in his mouth during the day time! Have it in front of him during tummy time or when he's sitting up and practice reaching for it, then once he's got it grabbed, help him figure out how to put it in his mouth! Baby steps! lol! pardon the pun! :0
2) teething tablets and/or baby orajel at night before you lay him down can help ward off the pain! (some might say tylenol, too)
3) put one paci in his mouth, but leave another close to a hand in his bed, so that he doesn't have to search long and hard to find one!
I'll pray for a restful night sleep for you and baby soon! I know it's hard, but it does get better! :)