Even hatchlings out of their egg do not eat fuzzy mice they are started on hopper mice and quickly move to adults, he is obviously being underfed which explains his lack of growth.

Solutions:

Simply feed a prey equal his girth size (largest part of your BP's body) every 5 days and when he starts catching up you can feed once a week.

As an adult if you have a male a prey between 55 to 75 grams weekly will be more than enough.