Divorced with a 2 year old

Ive been divorced for almost a year. Me and my ex have joint custody of our son. I work early morning (4am) so I have to have our son stay with him. He works night shifts. I spend as much time as I can with my son but my ex doesnt think that its enough. I cant control my work schedule. I cant control the fact that my truck got cancelled today and ill have to work on my day off. It hurts when i have lose time with my son because my family also wants to see him. I'm working to support our son. I love my son so much and i hate that my ex thinks I don't.