I do think he would remember it even with out you remnidning him.
Just because it was so impacting upon his life even if he is so young.
It's funny if you talk to him about stuff that happened early on in the treatment (he had to spend alot of time in the hospital) the stuff he remembers best is the fun stuff, the playroom, the toys he got, stuff like that. He even remember it as the first place he played Mario on a N64 that they had in the playroom (I guess it runs in the famliy!)
And if i have heard correctly geting a bone marow test can be quite painful. Hopefully i heard wrong.
As you might expect he has had several in the past couple of years as well as about 15 spinal taps, the nice thing is the put him out for both procedures (mostly due to his age I think) He's never shown any signs that it's painful afterwards so we should be good.