The rule of thumb is 6 months old because you don't want them using up too many calories that they need for growing. I don't know who told you 2 - 3 months old, but there is no way to estimate a chins age, unlike dogs, horses, and other animals. Regardless of his real age I would be concerned that a rescue would need some time to adjust to your home, the food you're feeding, and put on some weight if they're underweight for their skeletal structure. If it was me I would give him several months before giving him a wheel.
I took in a rescue last May. Her new mom just facebooked me this week saying she was still putting weight on. She was in the 500g range when I got her, and this week she's up to 793g (Yay Willow for those of you who remember her).
Additionally I have a 7 month old from a great breeder. He's had some issues with weight, and even though we're moving in the right direction he's not going to be ready for a wheel for a long time.
I guess my point is don't push it just for your own enjoyment. Give him time, time won't hurt him, but giving the wheel too soon could set him back.