Drills

Drills to stop the stepping out:
Have him just pick up the front foot & set it back down (e.g., essentially no stride).
Put an object behind his left foot, positioned such that if he steps out his foot will hit the object.

Regarding accurate throwing, check his throwing mechanics first. Make sure he keeps his eyes focused on the target, and does not follow the ball's flight (as it is leaving his hand) with his eyes.

Grounder drills:
Roll (literally roll, keep them on the ground the entire way) easy grounders to him & have him use his barehands to field them. Make sure he is bending his knees & getting his backside close to the ground. This will help him start with his glove in the dirt (as will the work with the practice glove) and get him use to having the throwing hand near the glove. It will also help him develop "soft" hands.
Using a practice glove (see Baseball Express website or Kelley Gloves if you want to buy one, or fashion one with a modified ping pong paddle), roll grounders to him. Then hit grounders. Have him put his cap in his mouth (bite the bill, with the top of the hat up). This forces him to field the grounder in front of him where he can see it. The cap blocks his view immediately below him & just in front of him. This also makes sure he positions his head properly to get the best view of the ball.
Using his regular glove, put a bat on the ground about six feet in front of him. Roll grounders to him. He is to run up, set & field the grounder with his feet behind the bat but his glove in front of the bat. Again, this is to teach him to field the ball in front of his body, rather than beneath him.
Remove the bat. Roll grounders, then hit grounders. Start with easy grounders to build his confidence. Work up to harder grounders over a couple of weeks.
Get about 10' from him. Have him get in his ready stance. Quickly throw grounders far enough to his right to force him to back hand the ball. Make sure he is doing his crossover properly & that he keeps the glove slightly in front of the body where he can see the ball all the way into the glove.
Finally, hit slow rollers to him & have him "run & scoop". Make sure he is putting the glove just outside the left foot & watch his footwork as he transfers his weight to make the throw.