An injection every 12 weeks is absolutely 100% USELESS. I would be curious to know the logic behind that doctor.
Other than that, yes you can inject testosterone in your dick. Now, would it make a difference? NO. ZERO. As soon as you inject it it goes into the bloodstream passing directly through your lungs and heart which will feel awkward.
Over the years there have been guys injecting all sort of drugs into their dicks, of course without any result.