Can you give him the bag to hold rather than putting him in it? He might be happy just to feel the fabric on his face. Yasin still likes to hold his old wraps beside his face when he goes to sleep. If it makes him happy it might be easiest to take the path of least resistance and let him have it as a comfort object until he's ready to get rid of it.
If it works you can later cut the bag in 2 so that you have a spare - it makes washing easier and if one gets lost its not such a tragedy.