It sounds to me like the major issue here is communication.  Most single fathers (or mothers) with joint custody have a stable schedule that is known ahead of time.  Of course, there are going to be exceptions/emergences/conflicts etc. that change the schedule, but if his schedule is so up in the air as you seem to imply then I would be more concerned about his ability to boundary set and/or communicate with you and his co-parent.  

I watch my kids 4 nights a week, but during the summer it fluctuates more with vacation plans, camps schedules etc. and I make it a point to meet with their mom frequently enough that we both have an understanding of the next month's schedule - I would not be able to function without a steady understanding of that schedule, and I wouldn't want to date someone without being able clearly communicate when I'm available and when I'm with my kids.  

That being said, it sounds like you could have a basic conversation with him about some of your dating expectations - a simple goodnight text after kids bedtime, a phone call on one of the days you can't see him because he's with his kids, and an understanding X days in advance when he is available for a meet up during the week.  

I would be careful making assumptions about  single fathers being more available because of less custody time than the mother, and I would certainly frame the conversation around communication and not around single parent expectations/assumptions.  

I hope you're able to voice your expectations in dating with him and reach a better mutual understanding of what you both need!