Because the subject here is "He" and not "You".

In Polish we use a special construct to express this: „po polsku”, „po angielsku”. I's like French a'la, „in a Polish/English/... manner. It is probably the last place where old dative case of adjective is retained. We have: „po polsku” but „polskiemu mężczyźnie”, „po angielsku” but „angielskiemu mężczyźnie”. We cannot say „po psu”, but only „po psiemu”. This shows that only some well established forms of this ancient dative are in use today.

"Gadać" is just too informal.