Advice needed

Hi all, my first post. I am  new mum to an 8wk boy. He was never breast fed and has had a mixture of instant formula and now powered since birth. I have had him on aptamil comfort  for the last 4wks now and its  hard work. if he's starving he'll guzzle 2 or 3 sips and then cry, I wait for him to stop and then give him the bottle back. This can go on the entire feed. however at other times he may take the full bottle beautifully. He is fine with the sucking as he has a dummy. It seems he is in pain when taking the milk tho. Any suggestions on what to do? Or what it may be causing the upset for him?