my understanding of ftt is that it is diagnosed when a child consistently falls below the 3rd percentile for weight and/or height, or when they cross 2 or more major percentile lines in a downward trend. so, your son was 71st percentile at birth, and now at 2 1/2 months he is in the 9th percentile so that crosses the 50th, 25th, and 10th percentile lines in a downward trend. so yes, he does fall in the category if ftt. ftt is a symptom and not a diagnosis or illness though. once ftt is confirmed, they generally try to find out what the underlying cause is and do things to promote growth. i hope that helps and makes sense to you.
btw, here is a great, and easy to use percentile chart for infants up to 36 months old.
Growth Percentile Calculator