Does infant gaviscon cause constipation?

Doc and HV seems to think it doesnt and that gaviscon has little or no side effects...?!! However our 9 week old baby boy was prescribed it to treat acid reflux 3 weeks ago, and it has really worked wonders he's rarely sick doesnt cry and no longer writhes in pain for hours at a time. However, he now cant go for a poo!! He does go once a day normally, probably 5 days out of 7. But (sorry to be descriptive) it's hard like a pebble, PATHETICALLY small and it takes 30 minutes of straining , scarlet- purple face and his vein popping out of this head!!! It's awful i can't bear it. I'm sure it's also causing him to not eat as much perhaps cos he is bloated?? As he is up throughout the night and only drinking 1-2 fl oz at a time if we're lucky. He is on double doses as is 12lbs (advised double dose if over 10lb) have tried going on to single dose but gaviscon is uneffective, but still no change in poo. Is it worth putting up with the refluxz for a while or is it really not possible for gaviscon to be the cause of this? Please note we are using Cow & Gate comfort milk as well which is especially for colicky and constipated babies!