Where should the circumcision line on my upcoming sons' penis be?

Below the glans. The "tip" of the penis is the glans, the smooth part. The foreskin is cut around that, and then falls back to the shaft, allowing the glans to stand out with the rim of the glans visible (it looks purplish usually). Sometimes there is slight re-adhesion of the foreskin to the glans, but we don't get too worried, those usually relieve themselves with future erections.
Variable. The circumcision line is always under the coronal sulcus, which is the groove under the penile head or glans. Some surgeons place the circumcision line very close to the sulcus, others make the line much lower. Discuss this with the surgeon in acse you have any strong preference.