For me it's vowels and vowel combinations. Put them together in any given groups and they ALL make each other's SOUNDS! And if it's the "long a" sound is it A_e? ai? eigh? ay? ae? So, yeah. English is EVIL! I'd probably be a great speller in Spanish where everything makes one sound and if you hear a given sound you have one letter making it.

As for a three year old spelling 'jet' - that's AWESOME! I have seven year olds who can't do that. And there's nothing wrong with a little wrote drill. Ask him twelve times a day until it sticks and move to a new word. His pre-school teachers will love you! :)