not getting any better :(

my little boy is 10 months old he was born 9 weeks premature and since january he has been on meds for reflux as hes constantly being sick, this week we had hospital check up and he has been taken off all his meds. but when they weighed him he has only put on a pound in just over 2  months his weight gain has got less and less at first he was putting on 2 pounds a month then it went down to 1 now not even this. so they have now put his on high energy sma milk to try and bulk him up, i thought that once they were taking solids and sitting up and being on the move it would get better but this seems not to be the case, im really at my witts ends with it and im constantly worrying about it, is anyone in the same position or have any tips for me?