It is insanely difficult to watch ones child be in pain. Knowing that they are and you can't end the pain, knowing all the implications of what your child is going through is just added heartache. My thoughts are with you, your child and family.
My newborn had to get tons of invasive treatments 27 years ago. He is fine and ornery now, but just seeing the words "lumbar puncture" brought me right back to that day when he had to get one to check for a sepsis-which he had and overcame!! I only experienced 2 weeks of hell with that one (until he learned to ride motorcycles, but that is another subject).
I know cancer is a long term stress on you both. It won't ever really go away, even after remission. Hopefully that awareness will make you grateful for good things. I always hope that our struggles are recompensed by perspective to some degree. I don't know if that really happens, but I'm an optimist.
May your sons doctors have wisdom and compassion. I hope soon to hear that he is "in the clear"!