My little one was on gaviscon and it also made him constipated so we stopped it. I tried the Frida Windi and they’re amazing! Maybe give that a go before you call 111 or your GP. Also, are you massaging his belly, bicycle kicks etc?
My 3year old was on gaviscon when she was a baby, we had to give her premade formula as it acts like a laxative. So if you're formula feeding it might be worth a go to see if it gets things moving. We ended up having to give her a few feeds of premade every few days to keep her going in the end x
I would ask your gp for some lactulose xx
Not constipation as such but before solids my little girl only pooed every 5/6 days and this started from I think 4/5 weeks I had a gp appointment and thought I’d ask anyway and he said not to worry and that it’s normal for them not to poo everyday