Hey,

To get Haploid callus one can culture pollen grains.

This technique was discovered by Guha and Maheshwari and since then it is been adopted for this process.

the pollens are obtained from the anthers and are then cultured on nutrient media, this is where they develop ino callus which is then treated with auxin which promotes rooting in them.

I hope this helps.

All the best!