Most courts require a material change before getting involved in a modification, however, you may be able to show that it is in your child's best interest to modify the existing schedule if he is being adversely affected by less time with you or not knowing for sure how long he has with you or his mother on any given day. It might also be possible to ask your ex to work with a mediator to revise the schedule so that it works for all three of you.
To modify, you do need a material change in circumstances, which I believe can be demonstrated by the fact that you used to be able to work out the visitation times but are now unable to do. Of course, this partially depends on how long this has been going on, etc. but in short it sounds like you'd be able to modify for a more definite schedule.
Christine G. DeBernardis, Esq. (978) 777-5393