Girls, how do you correctly switch to mixed feeding? The husband bough…

Girls, how do you correctly switch to mixed feeding? The husband bought Gerber Gold and gave the son he doesn't drink today, I tried too, he doesn't want at all. A bottle of 6+ from Avent. We rarely gave formula in the first months when we went for a walk, the bottle was also from Avent, but the baby ate NAN. His face does this "??".