
why is he not growing?

They explained it I believe in chp 24 or 23, to summarize it, there are many benefits for Issac to gain whilst a newborn, there are many opportunities or abilities he gain whilst being a newborn, so in order to stay a newborn he has been suppressing his growth with the help of his body part that he swallowed
FIRST TIME I READ A SLOW BURN FREAKY COUPLE LMFAO