It doesn't sound good, but a picture would help.
The blotches may be down to insect infestation.
Thank you for your interest, Alina W. Hopefully this picture will help....
Doesn't look brilliant. In the winter you could try shocking it back into growth with some quite severe pruning, crab apples will tolerate this; but at the same time give it a good feed of general fertilizer and ample organic matter , such as compost to boost new growth. If that fails I think you will have to evict it.