I am so sorry to hear this (((HUGS)))
Flannel on the front, except for the spaces for the signatures, cotton batting and flannel on the back.
This quilt may need frequent washing and cotton batting may hold up better. You could use to layers of this batting :wink:
Poly batting doesn't wick, although you want him warm, it may be nice that the quilt breathes too.
High loft battings would also make it harder to sign the blocks.
JMHO :D:D:D