From the FAQs:
Any child aged three- four qualifies for free admission, however they will need to apply for a membership card to be assigned a seat.
Any children 2 years and under are classified as babes in arms and can enter the ground for free but will share the seat with the paying adult.
So best to get his free membership for 2019/20 so his seat is there for when you need to start paying for it the following season...the membership office are very helpful with all this stuff.
As is the fashion these days, can I simply identify as a three year old and get free membership?