Hey mike. You asked for advice, and you're getting it. I don't think arguing with the advice givers is a very good idea. If you want to get tested for testosterone levels, then by all means, go ahead and get tested.
But the prevailing opinion here seems to be, it's more your personality and character that are what cause your lack of aggressiveness. Not everyone IS aggressive, but self-esteem and confidence are things you can work on.
Martial arts is an awesome way to boost booth. Just working out will boost your confidence, probably faster than you are even seeing muscle gains. There are books, videos, councelors, to help you with esteem issues.
The guy who picks on you IS more afraid of YOU than you are of him. He also has LOWER self esteem than you do. He picks on you because he CAN. If you even BARKED at him once, I bet he'd back down. I mean that literally. He bugs, you, and you let out a giant 'WOOF!' loud as you can, right in his face, he'd jump 10 feet back, I bet. Not only would he be scared of your 'retaliation', he'd also think you are crazy, and believe me, it's smart to be scared of crazy people.
Bottom line is, if you want him to stop picking on you, you will have to do something, or it could carry on for a long time. If you stand up to him, the odds are very small that you'll have to fight him. And if he throws the first punch, ( which he won't ) hopefully, it will bring out the crazy in you, you'll beat the crap out of him, and your problem should be solved anyhow.
But please, lay off the low testosterone argument. If you want to persue that route any further, get tested.
Otherwise, please LISTEN to the advice given by the people that you asked to help you.