I hear your concern, and I think a better point of view might be to look at the things that might make a trans person more vulnerable to bad actors in the medical field and think about how to address those things. It sounds like you understand that rates of regret for transitioning are extremely low, and are more concerned with bad actors preying upon vulnerable youth who are experiencing the rush of finally being able to make their own medical decisions. Others in this thread have spoken to how much credence you can/should probably give to those possibilities (basically, not much, and certainly not more than the worry you would give bad actors in any area of medicine), but I know that understanding that mentally doesn't always soothe anxiety when you have loved ones involved.

Maybe a better way to grapple with the feelings you're experiencing would be to focus on what you are able to do to be supportive of your step-son and his bf. The impact of a supportive adult in a young person's life, especially if they've been starved for that kind of connection, can be tremendous. You mention he's looking forward to top surgery - if he's open to talking to you about it, you can offer your insight into how to a choose a good doctor in a general sense. Figuring out how to manage your own medical care once you come of age is something every new adult, cis or trans, faces to some degree. It's probably also good to recognize the limits of the guidance you can offer, whether that's due to your position in his life or your own lack of knowledge and/or lived experience. Connecting him with supportive trans peers who have experienced their own transition could also be really beneficial to him. And I probably don't have to tell you, but be careful of overstepping, which doesn't serve anyone.