Hi Catherine.  I have spent most of my sons life worrying about his growth, so understand your concern.  At 20 months though, it is impossible to know what the final outcome will be for your little lad.  If he is regularly monitored and his treatment is good, his bone-age should stay stable and his height shouldn’t be affected.  However, that is easier said than done and some kids can be difficult to manage.  If this had been the case with my son, it was reassuring to know that growth hormone or more recently aromatase inhibitors were available which have the possibility of improving final height. 

My advice would be to keep vigilant, question everything as you are your childs advocate and need to ensure he gets the good care and attention he deserves! An annual bone-age x-ray from the age of 2 years gives a good indication of growth control, as well as regular blood tests.