Sleep regression- long post

My LO is 12 months old and we just switched him to wcm. At the same time he suddenly is afraid to be apart from us (and caretakers) when going to sleep. He used to just fall asleep when rocking or after feeding or get drousey and we would put in crib. He used to wake for a bottle once a night and we would feed him and he would go right back to bed no issues or crying. The last 3 nights he screams and clings to us when putting him down. He literally will try to climb up us to avoid going in the crib. I have tried to rock him but his eyes spring open as soon as I get near the crib. Last night I tried sleep training ( let him cry and go into room after 5,10,15,20 min) and didn't pick him up just pat and lay back down. He seemed to get more and more worked up when I would enter the room and leave. After 1 hour he fell asleep with his head on the crib rails (break my heart!) he woke up at 4:00am and cried for 5 min. I went in and gave him reassurance and he sat there in his crib for 50 minutes just staring at the door. After 50 min he started crying and by this time it was after 5 so I made him a bottle. He went back to bed okay. Only 5 min crying. Any advise on what to do about seperation anxiety at night given this is new? Is it bad to try sleep training? I struggle with when to go in room and reassure verses trying to let him work it out on his own. Urg! Why now!