If my son is small for his age, will growth hormone replacement therapy be helpful?

Maybe. A pediatric endocrinologist would be the specialist you should see.
Depends. The final projected height of your son would be the average of dads height+moms+5".If your child enters puberty later than his friends & has a bone maturity(xray study) that is delayed, he will have potential to continue growing after others stop.If he has documented HGH deficiency then HGH shots could help. Hgh treatments are unlikely to increase final height in kids without a deficiency.