My little man (8 months tomorrow) was on the 25th centile and just slightly chubby to start with, but when he was about 4 months old started to lose most of that and dropped eventually to just above the 0.2 centile, while 50th for length. Now he has a smal head so that accounts for some of it, but he was simply very skinny, no tummy at all, double chin gone etc.
But he was ever so healthy, never ill, very very active and super alert, always on the go, so hv never was particularly worried.
Now he is on solids, the double chin has reappeared, more tummy than he has ever had and even some fat over his ribs. And still always on the go.
He is bf so no idea what goes in, but I would say relax a little. I wish that I had not always fretted so much. hv and others are always quick to pick up if there is a problem.
of course if you think he is still refluxy that is another matter, but the stressy behaviour you describe is what I recognise from trying to feed a baby who does not want feeding and who sensed my stress.