Introducing formula at 6 months

My son is breastfed but I’m struggling to express and produce enough as it is at the moment due to a new medical issue that has come up. Within the next few weeks I’ll be starting a new medication and won’t be able to breastfeed at all, does anyone have any advice on how to transition him? Will he get an upset stomach? Any formula recommendations? I’m gutted so want to make this is easy as possible for him :(