He probably was not trained. You'll need to spend a few hours with him; in a room with litter tray. Watch him - as soon as baby will start looking for a place to wee - take him very gently and place into litter tray. May be after he will leave a "mark" in litter tray and scoop litle bit - he could learn. May be you will need to repeit it a few times.
Also - many cats prefer to have separate trays for wee and poo.

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