Your son is old enough and feels a part of the dog enough that he probably needs to become 'OK' with the decision before you euthanize. yes you've already scheduled it and yes you should go through with it, but help him come to the understanding that you are not deciding if he will die - only when so that he can avoid the next few weeks/months of pain and the pain and fear he will feel while dying of the illness.
Keeping your dog with you now is only for you, a dog who can no longer walk has lost most of the reason to be a dog but will hang on for the family who loves him no matter the pain, so we often have to make the hard decisions for them and euthanasia will allow him to be surrounded by the people he loves when he passes on.
Anger is part of the grieving process so what he is feeling is normal but he might be able to heal faster if he can get past that and feel he has some control in the decision before it happens.
Having him with you is your decision based on how well you think he can handle it - if he needs to be reassured that there is no pain/fear associated with it, or he feels he is his dog and would want him with him, then it might be good for him to come along.I have watched my dogs die of natural causes and watch them be euthanized and in all cases the euthanasia was very peaceful but still heart wrenching.