I appreciate your concern for your grandson.
Since your little one was a preterm baby such pfo usually are not very rare.
What you need to know is how big it is and whether it will self correct or will need surgical interventions and whether the syndromes which are associated with this condition has been ruled out.
It is good that tube feeding has been stooped and baby is being fed by mom.
As far as the weight is concerned even in a normal baby there is weight loss
in the first 15days and then there is a gain in weight by approx 25-30gms in a day.So one will have to be a little more patient with this preterm baby and give time to the baby with cardiac problem to gain weight.
All the best.
Hope to have answered your query .