Thank you for your query.
I believe by ultrasound
of the heart you mean an echocardiogram
was done. One can have difference of opinion on the interpretation of the echocardiogram and that may be the reason that they are repeating it on your grandson to be doubly sure.
It is not very common to see repeated strep infection in a 10 month old child. If it is confirmed strep infection then one has to do a work up to find the cause of repeated infection.
I hope I have answered your query to your satisfaction.
Wishing your grandson all the best.