bramstow you could be describing my son!!
Same as yours, was fine until he hit 10 weeks then instead of taking a 6oz bottle he would only manage 2 -3 ozs, he would scream, writhe around and take ages to feed.
He had no sleep in the day really and then slept loads at night from sheer exhaustion.
He was diagnosed with reflux (he did spit back clear liquid with white specks in it over the paediatrician which shows that hours after his last feed he was still refluxing) he was given infant gavsicon but that did nothing he was then put on a special formula called Enfamil AR which we got on prescription.
You can get the equivalent thing, I believe it is SMA staydown - it is the formula you HAVE to mix with COLD water. You could try that to see if it makes any difference.
FILM your son screaming the place down and what he does so that you can take that with you to the doctors because it is not normal.
My HV didn't really care that I was feeding my son small amounts 10 - 12 times a day because he was gaining weight
Is he happier when propped up and screams when he lies down?
I agree that there could be a lactose intolerance issue also.
What are his nappies like? Does he do normal poos?
Will check back on this later