I would suggest that if it's at all possible (keeping it a surprise), to have the enclosure setup and working right before getting the BP. That way you can work out the kinks getting the humidity, ambients, and hotspot temps right. Then all you do is put the BP in and it should be good to go.
+1
I would even go as far as getting the BP a week or two early so it can get acclimated to its enclosure before your sons birthday. That way he will be able to hold the snake on his birthday without stressing it out to much.