How is the little lad ever going to own his own home if he's already eating Avocado?
Pretty poor parenting if you ask me.
The book is originally from the 80s, and if I remember correctly part of the conceit is that none of the family know what this strange "avocado pear" is (because in Britain of the 1980s no-one did). I don't think the financial ramifications are really discussed, and the family keep feeding the little blighter avocados until he gets strong enough to beat up a burglar or something.