If he's diagnosed reflux, why is he on infacol and movicol? Why nothing for the reflux, not even infant Gaviscon (which doesn't actually do much, btw, but is the usual first step!)? My DC1 was on movicol at around 10w but alongside domperidone, after Gaviscon did nothing and before moving onto omeprazole. I can't remember what DC2 was on and when because having a 2yo and a baby both with severe reflux simultaneously definitely fries your mind a bit - but at no time did they give infacol.
Both mine would arch and go rigid during and immediately after feeds, the screaming was awful too. The vomiting made a diagnosis obvious. Constipation went along with it, whenever enough actually stayed down it just didn't come out the other end.
What helped:
Holding upright, especially during feeds and for a good half-hour afterwards
Wedging the mattress up a bit - either prop up the legs (think the ones we bought were called "bed blocks", we still use them now as DC2 has asthma) or if still in the Moses basket, rolled-up towels under the head end of the mattress (also works in a pram)
Using a bouncer chair as upright as possible and with as little bend in the middle as possible (try to keep the stomach stretched out instead of scrunched up)
As little time in the car seat as possible - that made things much worse
Using a sling/baby carrier rather than a lie-flat pram/buggy
Walking up and down stairs after feeds - for some reason the slight bouncing motion seemed to help winding.
Cranial osteopathy. I was sceptical but desperate. However, after every single session, both DC would be totally relaxed and fall asleep for a good 90-120 mins. Dc1 only ever slept for 40 mins at most, Dc2 for 25.
I'm sure there were other tricks, will try to remember. DC2 now almost 4 and both only suffer occasionally from reflux. DC2 did have a CMPI too, so you could maybe try different formulas (I BF both, so just cut out dairy myself so I'm afraid I can't recommend formula, but I'm pretty sure the consultants were prescribing a formula beginning with E to the FF parents at the reflux clinic, maybe someone else knows the name? It is available on prescription, so perhaps worth asking?).
Hope you find something that works soon.