All babies are different. Some will eat more and go longer between feedings others may eat less and eat more often. Yours sounds like the later.
He could be arching his back and getting fussy because he is gassy or he could be telling you that he has had enough and to stop trying. If it were me I would pause the feeding, burp him, lay him on his back and bring his knees to his chest, and mesage his tummy in clockwise circles. All these techniques can help remove gas. Once he seems comfortable again try feeding more. If he doesn't want it he doesn't want it.
The truth is most bottled fed babies over eat. Parents tend to look more at the ounces rather than baby's body language. Mothers of breastfed babies can only go by baby's body language. Trust that you son knows when he has had enough and expect his eating habits will change over time.
In response to why is he eating less. Perhaps he recently went through a growth spurt where he was eating more. If he growth is currently a little slower he won't need to eat as much. I wouldn't worry as long as he isn't loosing weight. I think babies this age should be drinking at least 20 oz of formula in 24 hours. If he is drinking much less than this contact the doctor.