My Son Isaac who is just over 6 just had one last week. they were so great with him.. There is honestly not much you can say to him at home but just not show your anxiety.

We just said you are going in for a BT in a few days.. He asked what for and we said to see if they can work out why he keeps getting migraines. We also told him it would feel like a little pinch (which is true) and also told him cool stuff like how he will get to see his own blood. And then said I wonder if it will be green or pink

Also for Isaac they used like a canula thing but in the same spot adults get blood taken from. they said it is easier then holding a needle in place as they needed 4 vials full..

Good Luck, I hope he is good for you. Does he have a special blanket or toy he can take to make him feel braver?