Toddler cries for mom in middle of night

My 2 y.o son cosleeps with dad. I cosleep with my daughter in another room. My son has been sleeping with dad for about a year now with no problems but lately in the last month or two he wakes up in the middle of the night and cries for mom. Dad cannot soothe him and he will not take a bottle or go back to sleep unless I go in the room and do it. He has no problem doing the night time routine with dad but doesn't want dad in the middle of the night when he wakes up. What should we do? Is this just a phase? Thanks!

-Tired momma