We have a child with this issue. She has an autistic spectrum disorder, and we chaulk it up to part of her anxiety related to her autism. She may ask us several times a week if she is going ot have a shot or a blood test, and any visit to the doctor or dentist is riddled with anxiety that at any moment, she may have to have one. For her, it is counter productive to "prepare" her when she will need one, it is much better to not tell her until it is time to have it done, less time to stress about it, get it done quickly and get on with making her feel better when it is over. Asking her if she is ready and telling her that it is not biggie, or that she needs it, or anything other than, I am going to give you a shot, here it is, now it's over...see, not a big deal, how about we go do something fun now, just prolongs the worst part, which is the fear before hand. There has never been a shot or a finger stick or a blood test that was too terrible, it is the anxious time before with the worry that you need to minimalize.
We put a big note ont he front of our kids charts, so that the staff did not dilly dally about it, did not ask if she was ready (she will never be ready, and since she does not have a choice in the matter, they should never suggest that they do by asking anything) Just get it done, short and sweet, no prolonging the anxiety. If your doctor's staff cannot do this, find one who will.
Mediations have helped our daughter, but unless it is a constant debilitating fear (like it was, as well as other things) then that will probably not be needed for your son.
We have a deal too, if she ever needs something, she has to let the person do what ever it is in the doctor or dentist office without moving, she can cry, she can screem, but she has to cooperate. If she does, we go to the toy store afterward, and if she doesn't...maybe next time. We have had this deal for years, so we don't have to alert her that she gets a reward, or what she has to do to get it before hand, which would just give her more anxiety. Once you get that established, it might help him be more cooperative, and telling them that they can cry or yell, gives them some control.
M.