It's up to you to teach him

After Reiner and Bertholdt (or however the hell you spell his name) brought back Ymir who was the holder of the Jaw Titan, the inheritance ceremony was conducted immediately to waste no time and ensure that Marley had the upper hand against the dreaded Eldians in Paradis Island. Of course, Marley military knew that when a Titan Holder transforms for the first few times, they may have difficulty controlling their Titan, hence why Zeke put you in charge of teaching Porco.