This is just for fun...I'm not being serious...but let's start him off with a good beginner cycle of Test-e (which you have).
I let's say the average male who is going to start using AAS would be around 200lbs; and he would start with a dosage of 500mg/week.
500mg/200lbs = 2.5mg/lb
So...if you rat is say 2lbs, you could try 5mg/week...or maybe 10mg/week.
But hell, you may as well front-load with 15-25mg/week to start.
Give him lots of food, and a big cage/area to run in. You could even make up a controlled course (which would be challanging for him) and have a trail of food to keep him going.
WOW. Sorry...quite the rant....