General feeding advice

DS has reflux and is on gaviscon, 3 x day in his bottles.
He's 13mths now and I hear that a lot of babies are going down to 2 bottles around now.
We tried 1/2ing his gaviscon a few weeks back, but it didn't go well so we put him back on full dose.
Under 1yr he was on S-26 step 2 (std, not gold).
In the last few weeks we've had him on sometimes dairy milk, sometimes S-26 toddler gold. He's gotten so constipated I don't know what to do, i've been feeding him prune mash twice a day.
When he was on S-26 step 2 + gaviscon we never had issues with constipation. He's not good at drinking water, fussy wee man. I've been through about 6 sippy cups/bottles before I found one he'd even consider drinking water out of.

For the moment we're putting him back on S-26 step 2, and hoping the constipation will clear up over the next few days.

I'm not sure if I should blame the dairy or the S-26 gold for the constipation. I'm not sure if I should drop a bottle, or which one. Then I'm not sure what to do the the gaviscon as I'm pretty sure he's not ready to have it reduced. And I'd be worried he's not getting enough liquid per day if he was down to two bottles.

Any advice welcome

You could still do 3 bottles a day just reduce the amount in them.