Well, I'm on baby #3, and to me it just sounds like a phase. Around 6 months is another (yes...another lol) huge developmental age. At 6 months you can start feeding solids (its recommended to wait till 6months), so if you haven't already, you can start on some food if you are worried if he's getting enough nutrients, however even at that age they don't really pick much out of the food anyway, and still depend largely on formula or breastmilk, but his interest in food is most likely there. He also may be self weaning (my son did this at 8 months old) but 6 months may be too early for a that, depends on the baby, I suppose. I would suggest maybe try offering formula a different way, so if you do it warm, maybe try cold, or Luke warm, or put it in a sippy cup (my so got to the point where he refused the bottle and only wanted sippy cup, unless night feeding and my daughter only wanted to drink out of a cup, refused sippy cups or bottles) . At 6 months he is probably more alert about the world around him, and may not want to be bothered by eating, all my kids have gone through that stage. Or maybe it's the growth spurt , which happens at 6 months, and usually kids have different eating habits around that time as well which may increase appetite(like my daughter) or decrease it (like my son)... sorry this reply is really all over the place and probably isn't helping haha. I'd say, keep doing what your doing, keep offering it, maybe change it up a bit, and maybe bring it up with your doc the next time you see him. As the famous phrase says " this too shall pass", and nothing could be more true to motherhood haha keep up the good work, I'm sure this curve ball will end soon, and then another will come ha
good luck!!