When my son was born and right up to this day he had a rather large penis.  He was in the hospital for 28 days and they had so many lines and contraptions going into him that he couldn’t really keep a diaper on very well.

Whenever the docs came by for rounds they made sure to display this part of his body.  His testicles did not seem to be an issue though.

If warning them, talking to their obgyn or ped is too tough for your family, watch over him carefully especially after day 9 or 10 because the baby lives on the mom’s antibodies, etc. until somewhere between the 10th and 14th day. 

If around this time he begins to show signs and symptoms (vomitting, lethargy, sleeping a lot) then you must step out of your discomfort zone and take action.

I do understand your hesitation with your family.  I went through the same thing too with my sister (unfortunately she lost the baby before it was born) but remember (and this is one of my favorite phrases) "there are NO do overs" in CAH.

I wish you the best of luck.