18 months is VERY young!! His brain has not matured enough to deal with the demands of being broken in, but most importantly, his spine has not fused at that age, so even a very light person sitting on him could cause severe and irreversable damage. Particularly if he went to buck, this could cause him severe injury.
Why the rush to ride him? If you brought/bred a youngster, you have to be patient enough to wait until they're mature enough for breaking, and when they ARE broken, you're looking at another couple of years before you can start any serious work with them. Young horses are a long and slow process.
in the meantime, you can mouth him if you're desperate. Rug him, through saddles cloths on him, do as much groundwork as you possibly can with him, think that you're 'bombproofing' him so walking him through all different situations. Float train him, take him out to places to let him look around etc. There is a lot you can do with them until they're broken.
But the most important lesson he can learn, is just how to be a horse, and to fit in with herd dynamics. He should be paddocked in with a group of horses and allowed to 'just be a horse' before you start drilling him with education.