The treatment won't neccessarily knock him for six. DD has been well for probably 90% of the the last 9 months. The doctors got on top of the anti sickness meds by the second round of chemo (she didn't have any sickness with the first dose). She's been in hospital with febrile neutropenia maybe 5 times (not counting the high dose, she's been spiking since yesterday evening), and she took a while to recover from her second op. Other than those times, she's been just like any other 4 year old.
I know what you mean about the GAs! This time last year we were agonizing over whether to put her though a minor operation to correct a squint (unrelated). Now she has a huge scar across her stomach, a scar under her arm with a rib missing, a gastrostomy and a hickman line!
The waiting is definatley the worst part when you're anxious to get on and start fighting this horrible illness, and to know what you're really dealing with. That's the point when your imagination can go wild. Once you start treatment, I found that you only look from one block of chemo to the next and from one scan to the next. And that's how I managed to miss the application deadline for DD's primary school place by 3 months...!
If you haven't already, get a diary that is just for your DS's treatment. We write down exactly what meds she has and the time, blood counts, transfusions, etc; any symptoms (being sick, rashes) as well as appointments, and questions we want to ask, phone numbers etc. We also use different coloured ink for when she's at home and when she's in hospital (makes it easier to see at a glance when we were last in).