يَا أَبَتِ إِنِّي أَخَافُ أَن يَمَسَّكَ عَذَابٌ مِّنَ الرَّحْمَن
...
O my father! Verily, I fear lest a torment from the Most Gracious should overtake you,
"because of your associating partners with Allah and your disobedience in what I am commanding you with."
...
فَتَكُونَ لِلشَّيْطَانِ وَلِيًّا
so that you become a companion of Shaytan.
This means, "there will be no one who will protect you, or help you, or assist you, except Iblis. However, neither he, nor anyone else, has any power over the outcome of matters. Following him will only cause you to be surrounded by the torment (of Allah)."
This is as Allah says,
تَاللَّهِ لَقَدْ أَرْسَلْنَأ إِلَى أُمَمٍ مِّن قَبْلِكَ فَزَيَّنَ لَهُمُ الشَّيْطَـِّنُ أَعْمَالَهُمْ فَهُوَ وَلِيُّهُمُ الْيَوْمَ وَلَهُمْ عَذَابٌ أَلِيمٌ
By Allah, We indeed sent (Messengers) to the nations before you, but Shaytan made their deeds fair seeming to them. So he is their helper today (in this world), and theirs will be a painful torment. (16:63)