Welcome you for query in this platform of HCM,
You have not told about the exact weight he has gain in this 15 months.
Your One Sentence that `He is Growing good' tells us that he is accepting sufficient required amount of calories.
There should be some weight loss
( failure to thrive
) instead of growing good if the vomiting
What ever he is vomiting that is extra for his body, it could be because of over enthusiastic feeding of the child. This is very natural, as you may be quite extra couscious about your child because he has suffered a lot in this very small life span.
Try to add some antiacid medicine like lansaprozole etc for some 2- 3 months period with consultation of your pediatrician
And please also know from your pead that how much you should feed in one go.
Hope Your query is better answered.