Not sure it will stick to mortar alone, personally I'd recommend removing any loose material, then getting a bottle of SBR and some cement and mixing to form a thick paste, then pour some in the hole and paint some on the slab back, then push it back in place.
Tips are don't walk on it or allow it to get frosted for a few days, also don't spill or drip it anywhere as it will stain, wash it off straight away.
You may be able to get a small amount of proprietary paving bonding that's ready mixed, to save buying in bulk as the qualities i bought in would cost you 30 quid and you'd waste most of it.
Good luck.