you have to do the following.
1-check with the cardiologist whether coarctation dilatation is still ok and enough blood is flowing through the site of coarctation..
2-the child had developmental delay for which there are many causes-the treatment is only continuous and vigorous but gentle physiotherapy
. Learn and observe from a physio and do more frequently whenever you find time. I suppose you already had an opinion from a pediatric neurologist
3 -For weight gain you have to give high protein and high calorie diet.
All the best