Yep, genetics are genetics and if he has poor hair genes then you're kind of out of luck. MTG won't do much for hair growth except moisturizing the hair follicle. However, it will attract dirt and grime in the summer and it will cling to dead skin cells in the winter, making the horse itch and perhaps rubbing the hair out again.
The best thing you can do is a weekly thorough scrubbing and conditioning of the mane and tail. Don't brush it, just gently pull out the tangles every day with your fingers. Use a spray on detangler - but do NOT use show sheen! Show sheen contains a butt-load of silicones and that dries the hair out. It might look shiny and nice but it's not. Use Healthy Hair Care Moisturizer. It's in a concentrate, smells amazing, cheap and it moisturizes the hair really well. I think the Eqyss Marigold spray moisturizes as well, but it's $25+ per bottle, and don't even bother with the Eqyss Avocado Moisturizing spray - it SUCKS.
Also, really massage the crest of the neck when you wash. It stimulates blood flow and lets more nutrients get to the follicle. You can put your horse on a biotin supplement as well. It's a safe water-soluble vitamin and your horses hair will grow like crazy. Farnam sells a cheap biotin-only supplement and it's the only thing I'll use... but it makes feet grow quicker as well so you might need more farrier visits.
I've got a lovely bunch of Neuticals,
There they are all standing in a row
Big ones, small ones, some as big as your head