Try telling him that you have to go to this event, but would rather be with him and then ask him if he can pick up son. Instead of just coming out and asking him to pick up son. Let him know that you would rather spend time with him but because of obligations you have to be somewhere else.
Can he go to these events with you? Also try stating something like that when he asks you about him not going. Tell him that you would really like him to go with you, but if he would rather not, that you understand. Then he knows that you want him there, but you are not making the decision for him.
