Hi, I had two with reflux (lucky me)!
It sounds very typical of reflux.
There could be two reasons for this.
You might find he will fluctuate with times that bother him. So just now it will be 7pm then a few weeks from now it will be, say, 3pm then 7pm then gradually build to become most of the day.
If this happens it means gaviscon alone is not enough and go back and ask for Omerparzole also. I found splitting the dose twice a day rather than giving it once works better with Omeprazole. You can continue with gaviscon as they are doing two different jobs. Gaviscon soothes like an antacid, Omerparzole lowers the amount of stomach acid being produced.
Basically what happens is, the more milk that goes in, the worse it gets and he brings up milk (either in the throat or completely out) with stomach acid and the throat becomes slowly inflamed thus 7pm he is just too sore to take that feed.
Secondly, he may be needing a smaller amont at around 3pm or whenever the feed before is so make that a slighty smaller feed.
I suspect the first one is the problem though.
He will grow out of it and it will all be forgotten before you know it.
You will see a huge improvement once he gets to 6 months then it usually goes around 1 year. Good luck. x