Hi,
I also would not start solids yet, its way to early in my opinion- the world health organisation also only recommends solids from 6 months.
My daughter also has terrible reflux and the Losec helped quite a bit, she grew out of it around 4- 5 months old and we started her on solids about a week or two before she turned 6 months.
You son was also born early, so I would wait longer for his digestive system to fully develop
Good luck