You won't stunt his growth hun, that is a myth. They will grow to their potential regardless of what they are kept in. Feed him correctly(Insects with a small rodent every 2 weeks or so) and you will have a healthy, non-obese, growing Savannah.

For a baby, try to keep the enclosure smaller and simplistic. If he/she is eating, shedding and growing, then the enclosure shouldn't be too big. But if they are hiding alot and not eating much, you may want to provide a smaller enclosure. Babies probably feel pretty vulnerable.