Polish, like many other languages, is far easier to learn to read than is English. In Polish the sounds which make up the spoken word usually have only one way to represent them by letters.
Unlike English which often, very often, has more than one way to spell a sound, e,g the /oo/ sound as in in 'boot' can be spelled 'ui' - suit, 'ou' - you, 'ue' - blue, 'ew' ' blew .... or, one letter or combination of letters can spell more than one sound, e.g 'oo' spells /oo/ in boot, but a different sound in 'book' (though this can be dependent on regional accent, before anyone says that they pronounce it the same way in boot and book.
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This makes English far more time consuming to learn to read and spell. So we start teaching it earlier because we have far more to cover to achieve skilled readers.
Do tell your son not to worry, OP. His child will learn fast once teaching reading begins.