I personally think your snake is way underweight for its age, I like to see between 50 and 100 grams per month of weight gain for the first year from my baby ball pythons.

I would get your snake off of mice and onto rats ASAP.

If you can't see the body of the mouse or rat inside of the snake then the food item is too small, don't be afraid to feed more than one mouse or rat at a time.

It's not uncommon for me to offer my babies that are less than a year old a rat every 3 to 4 days.

Best of luck

Brian