My first instinct is that your tree is suffering from apple scab. Here is an article from Perdue University. It includes information on signs and symptoms as well as treatment.  Here is another article from MSU with photos of leaves to help you diagnose your tree. This article talks about using fungicides to prevent scab (timing is key--if you spray too late, it won't work). 

Make sure that you clean up all the leaves from this tree and any other apple or crabapple trees as soon as they fall. You don't want them sitting around, reinfecting your tree. 

Alternatively, your tree could suffer from cedar-apple rust (or a similar fungus in the same genus). Here is an article about it from MSU Extension. If you scroll to the bottom, you can see a photo of a leaf from an apple tree. Does it look similar to the pattern on the leaves of your tree?

If you have redcedar trees in your yard, you may want to remove them. It's not a guarantee of prevention, but there's no sense having a definitive fungal host so close by. Here is more information from the Forest Service.

The other option is to keep spraying. You need to find a fungicide that says on the label it is for the control of cedar-apple rust and you need to follow the label precisely. 

These are diseases you cannot get rid of. You can only try to manage it. You may decide that the better option is to remove and replace the crab apple with a tree that is resistant to apple scab and cedar apple rust. You can see a list of apple scab resistant trees in the Perdue publication above. You can also look at this study from The Ohio State research station as Wooster that shows results of their study on most resistant crabapple selections.