Generally, having a bone age BELOW his calendar age is a good thing in the early years--it gives you some wiggle room going into adolescence to keep growing. Our daughter was way below the 5% (bottom) of the growth charts until she was 5, then had a sudden unexplained growth (and bone age) spurt. About the blood test--Why don't you call your doctor back and get more specifics about this new test--the proper name of it, what it will specifically track, and so forth. Keep asking until you feel comfortable with what is going on. laurie