Hi & Welcome.
'Innocent murmur' does not appear to be the cause for failure to thrive.
Also called as functional, physiologic, or benign, these
murmurs are normal, with healthy heart and are harmless. Innocent murmurs may be heard in children because their hearts are very close to their chest walls. Sometimes
anxiety , stress, fever, anemia, & overactive
thyroid cause innocent murmurs. An increased amount of blood flowing through the heart is responsible for the development of innocent murmur. These murmurs are unlikely to be the cause for Failure to thrive (FTT).
On the other hand, heart murmurs that result from a valve defect, narrowed blood vessel, or other cardiovascular defect which may have been present since birth (congenital) or developed as the result of another medical illness are called as 'pathologic' heart murmurs, may indicate the presence of a serious heart defect, especially when accompanied by other signs and symptoms of a heart problem such as
shortness of breath, rapid heartbeats, or
fainting. Failure to thrive (FTT) is an accompanying symptom of such murmurs.
Your grand son needs to be further evaluated to rule out other conditions such as anemia,
hormonal disorders, genetic diseases, or diseases of the kidney,
malnutrition, etc.
Hope the suggestions given above would be useful.