If he had provided the time/place where the event is taking place, it appears that this is an open invitation. You've already followed up twice. It would be best to let him reply as it should be him communicating with you. If you're finding yourself frustrated and him not very communicative, you may want to at some point talk to him about this situation.
If he does not respond by the time of his party, you can still decide to go or not. Calling him again won't change anything. If you are the only one to constantly call, even if it is to remind him that he had not gotten back with you, you enable this behavior. ADD should not be his excuse.
Thinking objectively is the ability and willingness to view a situation without attaching emotions or ego desires to it that are suitable to the self. Just like any neutral person who had not invested feelings and has no expectations of him, you too see the situation for what it is (without rationalizing or excusing his behavior in order to stay peaceful)