Refusal to finish bottle after burping, help!

[Mamia1552]

My nearly 4mo used to drink five 6oz bottles every day but now I'm struggling to get even 3oz into him per feed. He acts like hes hungry initially and takes the first half of the bottle no problem. After winding however, he screams and shakes his head from side to side and refuses anymore. I'm stressed at the thought of him losing weight (hes not chubby and can't afford to lose much).
I don't think its colic, hes in great form except for when I'm trying to get him to take the bottle and I can't see anything in his mouth (thought it might be thrush/ sore throat).
Any ideas?

Could he simply be full? Sometimes their level of hunger and intake does vary, particularly during growth spurts for example. How long was he taking more for and how long has he been taking less? Does he still have lots of wet and dirty nappies? Is he well in himself? How often are you weighing him and could you do so for reassurance? Is he perhaps taking milk more often?

I didn't FF but I do know my DD seemed to want more or less milk week to week and still does with both milk and solids.