dom, welcome!
The blockers and HRT should stop any further masculinization for your son, including the appearance of new facial hair and changes to bone structure.
Unfortunately, existing facial hair is not affected by blockers or HRT. The same is true for the size/shape of the larnyx.
However, many trans women find that blockers and HRT reduce body hair significantly over time.
Facial hair can be lasered away if it's dark, or removed by electrolysis if not.
And even a deep voice can be trained to a female pitch, resonance, etc. There are also voice surgeries that can help.
Also, the younger someone is, the more effect HRT is likely to have on feminine fat distribution, breast growth, etc. The changes for teens can be striking.
There is a ton of information on this site about transition, and people are pretty good about responding to posts. Ask away...