Well, that's good news about the "anemia". Glad you have a well-informed doctor.
Has J ever had to do blood draws before?
You might want to call the lab where you will be going for those tests and ask them if they do anything special with children who are anxious about the blood draws. Here they will sometimes use a numbing cream called Emla to help take away most of the sting. It's a pain to use though. A heavy layer of cream has to be applied where they think they are going to draw the blood and then covered with a clear plastic bandage. It takes about 30 minutes to work and then the cream has to be wiped off before the draw can be done. So it prolongs the process which, if you're going to place an IV is OK but for regular blood draws it makes it take much, much longer.
He may take his cue from you. If you are able to be calm and matter-of-fact about it then he may do better. If you have any bloodwork that needs to be done, you might go at the same time. You could go first and show him how easy it is and that might reassure him.
You might plan a favorite little outing or treat to do after the blood draw but don't expressly connect it to doing the lab well. Just say well let's do this when we're done so he has something pleasantly distracting to think about. If you are going to get a treat like ice cream, when he's sitting there just strike up a casual chat about what kind of ice cream he will order or does he remember the last time you were there and you laughed about the silly little dog - anything pleasant but kind of low key that will keep his mind off what's happening. Sit so he can look at you when you're talking but has to turn his head away from the person doing the blood draw if you can. It is better, in my opinion, if you don't try to hold him on your lap which I have seen sometime done here.
You could try a small reward but I wouldn't do anything too big because you don't want to set him up for thinking that he deserves a big reward for this.
Last suggestion - you could give him a strong scented gum (Juicy Fruit works well) to chew right before he has the blood draw. The strong scent will distract his brain from the pain believe it or not. But it has to be timed very close together.
Good luck. I expect it will be fine. My son has had high glucose readings a couple of times and when we repeat the labs it is fine.
Patricia