Letrozole is an estrogen blocker per se, prevents the conversion of the body's male hormones to estrogen and therefore slows maturation of the bones,It is used in breast cancer and tends to lower the body's estrogen and may raise testosterone levels, it is sometimes used with growth hormone. however it can also affect the liver and use of it is controversial in regards to its true benefit. A bone advancement by 2 years is very very mild and if this has remained and not advanced beyond this over time he is even better off than those whose bones advance even more than their age with time. A predicted height of 5'8" sounds ok but needs to be assessed in comparison to his midparental height, is this genetically appropriate and if so I would not go with treatment, if you are on letrozole then chances are you are in the hands of a pediatric endocrinologist who has experience. I tend to use it rarely-1-2 patients in our entire practice to block the effects of estrogen in girls with a rare kind of early puberty.
Adrenarche is what you are talking about, the adrenal gland kicks in a little earlier than normal giving us some expression of normal but annoying male hormones causing body odor, pubertal hair growth and some advancement of bone maturity, however, obesity itself can advance the bones as well and I would seriously look at the weight to be sure that the weight is controlled well, rapid weight gain and increased insulin production can also stimulate bone maturation and also adrenarche is influenced.
I would make your decisions along side your pediatric endocrinologist, they have worked with you and your child for 5 years now and know him better than I do, they know what is best and can best answer your questions.