E.B.
I don't have personal experience with it.
But if you aren't already seeing a pediatric specialist, then I recommend you do so and get a second opinion. Not because your first doctor might be wrong, but simply to get more information about the treatment, the side effects, the lifelong effects, what to expect, etc.
And as another mom suggested, make sure your pediatrician is doing a full gamut of tests to see if there is an underlying disorder which needs to be treated, and which may be contributing to the slow growth.
Premature babies, as you know, can have developmental issues that are not a result of underlying disease, but rather from simply being born early. If that is the case with your little guy, the hGH may be the thing that is best for him.
It sounds like you are a great mom, and are helping both your kids to be the best they can. :)
Good luck.