Mix feeding?

I started to give my LO formula milk (we tried 3 diff formula as per pedias advise enfamil gentlease, similac tummicare & now hipp organic 6-12); since his pedia advised me to start mix feeding. He said that my milk supply seems to deplete since my baby’s ht&wt does not seem to increase like before. But currently after many attempts of giving my LO formula milk my LO does not want to take it, i already tried giving it to him in a bottle, cup etc..but he still prefers to breastfeed. Did anyone experience the same thing? And my LO also just started his complimentary solids which he enjoys. Any advise?