Talk to your doctor, unless he is ready to sit you on the couch for the rest of your life. (If so, get a new doctor.) A good doctor should support you exercising, and if you have had a lot of muscle loss or atrophy, should be open to seeing you do what you can to help. But testosterone injections might do more harm than good.
Is your son a physician? Congratulations on attaining your age of 88+, however without a proper checkup by a physician with blood tests, I question your need for testosterone injections. "Seratulin" is an unknown to me. I also wonder about your need for a personal gym trainer and building muscle at this stage of your life, depending upon your overall health and whether you have been athletic and physically active in the past.