First, let me preface that this is not legal advise, as we do not give legal advise on this site.
The IDEA does not say anything specific about being late to meetings (to my knowledge), but it does speak to parents not making themselves available for or replying to attempts to schedule meetings. At some point the school has to move on and just make sure all attempts to schedule the meeting have been sufficient and documented. I would say being significantly late to meetings would fall under this provision.
However, the ADA might apply here requiring accommodations for dad's disability. But it sounds like you are doing everything you can. Can you talk to the dad about this issue? Ask him what the best way to get him there on time would be? Ask if he has someone who can assist him? If he is not working, he might be getting assistance from a government agency - not just financial, but also daily living. Could they help? Do they make sure he gets to doctors' or other appointments on time? Who is making sure the student gets to appointments on time, because it sounds like he wouldn't be able to. Can that person/aide help? I personally think booking longer timeslots would be an unnecessary burden on the school.