Jack started getting his at 13mo (they went for 5 months - 2-daily, then weekly
)
First of all, I'm a firm believer in asking for Emla cream for little ones. The hospitals have huge supplies of them. If they don't have it though, just make sure you pin him down well, they can move amazingly quick and hard if you haven't got every extremity tied down.
Sometimes it's easy, sometimes not. Jack always had issues with getting enough, so they had to prick him a few times each visit. I always found the hand the best, although it's more painful.
As for how much, they usually take 3-4 vials for a general screen, as they need different amounts for different tests (some tests can be done together, some apart). We always went for the magic 4ml for neutrophils, as sometimes they took less which meant we'd have to go back for more bloods.
Most places are great with dealing with kids if they are a bigger place - lots of toys, cool stickers, lollies, whatever works. I hope that it's something simple and he doesn't need too many