I haven't done this but I know many women who FF part time. My aunty went back to work full time when her son was 8 weeks old and he was FF all day during the week but BF overnight and on weekends. I would think by this stage you have established a good supply so missing one feed a day for a few days a week wouldn't muck up things too much... but I'm not an expert so would hate to tell you it's okay for sure and then it's not. I guess the thing to be aware of is most babies have a growth spurt around 4 months and many women claim they "dry up" at this point, but in actual fact the baby just needs to feed more often and boost your supply up to another level. If you are mindful of this and feed on demand when with your son I'm sure you would be fine.

I do know many babies who refuse formula so if you are thinking you'll get to a point where you HAVE to use it then perhaps try and start introducing it slowly to get your son used to the taste and ready for the switch.