I experienced back pain around 2 months after transplant. CT showed nothing on the spine, and the doctor really didnot know why my lower spine was in so much pain.
The back pain lasted for 2 weeks and subsided, reason unknown. My back is fine now.
I hope your Father falls into this kind, and the pain will subside.
Female 54, diagnosed MDS RAEB1 in August 2016, watch and wait for 9 months. Sibling match - Stem cell transplant in June 2017. Some GVHD skin.