With Ben you have to be in the left lateral position for the lumbar puncture to be made, I would ask you to lie down facing the left side of him and leave your legs bent, with your knees towards the abdomen.
I would explain to Ben, that this is the correct position he should be in, because the lumbar puncture is performed through a needle that will be inserted in the spinal canal that is in the lower back, for this reason, he should lie down in this position that leaves the back accessible to the doctor who will perform the procedure.
This also explains why the doctor would not want Ben to be in the supine position, because that position would make Ben support his back completely on the bed, preventing the doctor from having access to the spinal canal.