I use the „Harwurzel Liquid“ by Effol 🙊🧡
BioMane supplement works wonders😍
Some horses naturally don't grow long hair, there's not much you can do about it... you can try paying extra attention to his feed, and add supplements for hoof and coat. Don't brush too much as this pulls out too many hairs and cause breakage.
Use a good conditioning spray so there is less breakage, you can also try adding keratin or protien supplements to your horses diet