The colt should be weaned at the age of 6 months. And by weaning, I mean that he needs to be out of both eyesight and ear shot of his dam, and PERMANENTLY. You need to make the separation as abrupt, complete, and final as you possibly can, so that the colt's instinct to nurse will die out quickly. And the colt has no business being in with a stallion, not now, and not ever. Stallions who aren't herd sires will often KILL foals, Mimi. And if you and your grandpa don't know that, then you shouldn't be in the breeding business.
The normal age for a herd stallion to drive out his colts is when the colts are around 2 years of age. At that point, they are old enough to sire their own foals, and to prevent inbreeding, their sire drives them off. They then band together into what are called "bachelor bands" for a few years, until they are old enough and strong enough to fight off another stallion and win mares of their own.