Yes, you can have testosterone levels tested. Unfortunately, testosterone "is" somewhat animal dependent and while you can have some stallions that have a relatively low testosterone level, be incredibly high libido, aggressive animals and vice versa. There are things that can be given that will result in testosterone suppression - essentially chemical castration. But by doing so, you may very well suppress things sufficiently that they don't "recover" from it and essentially remain/become a gelding. If someone is goign to go that route, we strongly suggest freezing them before the process begins as you may have nothing there after the process. And, if you're going to do that, you may as well freeze and geld.
Stallion AI services is located in the UK. Have you contacted them about possibly freezing him? They are good and competent and Tullis Madsen is certainly approachable and easy to work with.
And no worries. I'm questioned all the time ! Often by myself. It's wise to question anyone on the internet! Some come across as much more brilliant than they are. Where are you located in England? I can perhaps suggest some other locations to contact for assistance. My hubby is a Brit and we actually host DEFRA certified courses over there periodically.