It's called age.
39 years old and on TRT already? You're probably training too much.
Your major mistake was getting married and having kids.
He shouldn't be on TRT because the problem was overtraining. He just needs to let his system recover. If it wasn't all that there are some vitamins that massively help.
L-Arginine and Pycnogenol are fucking amazing they should be used before Viagra every time. They help thin the blood out, I didn't have issues with sex drive but that made a big difference anyway. There are other things too like Iron levels might be low, I also added Guarana because someone on here said it was good a few months ago and yes I felt that give me more energy.
To answer your other response, Dexter Jackson has trained 3 times a week for years. Dorian was 4, you think Dorian would look worse if he moved a few bodyparts to other days and only did 3 a week? You really don't need more juice or not, the body still needs to recover. The six days a week training is rubbish, it's all about what you do and the body needs to recover you can't just go hard every day, if you try to there's not real intensity there. I guarantee these guys who haven't taken a rest will be blown away by how much harder they go for weeks after the rest.