have him answer two or three very simple questions every day. do it by email and then u can show basic grammar, if need be.
have him watch certain youtube clips, which u suggest.
tell him to buy an 'easy reader'
have him buy a grammar book with all explanations in Polish. or get one sent from Poland.
give him loads of confidence and tell him he's doing great. otherwise he may give up.
find out what level of english he actually needs. old test papers etc.