My advice to you is to prove to him asking Prophet Muhammad is okay. Once you do that then move on to Imam Ali. Use this verse Nisa 64
وَمَا أَرْسَلْنَا مِن رَّسُولٍ إِلَّا لِيُطَاعَ بِإِذْنِ اللَّهِ ۚ وَلَوْ أَنَّهُمْ إِذ ظَّلَمُوا أَنفُسَهُمْ جَاءُوكَ فَاسْتَغْفَرُوا اللَّهَ وَاسْتَغْفَرَ لَهُمُ الرَّسُولُ لَوَجَدُوا اللَّهَ تَوَّابًا رَّحِيمًا (64)
Also, the fact that Allah tells us to pray in the Maqam of Ibrahim