Hi Welcome to Health Care Magic......
I would recommend the baby to have a neonatologist consultation. The blister
what you are describing looks like a meningocele
Meningocele is a thin menbrane covered fluctuant swelling present in the midline over the back in the lower aspect, looks like a large fluid filled blister.
Baby needs a thorough evaluation of his lower lumbar part of spinal cord
and his limb functions.