Hate to try and diagnose someone's swing problems without actually seeing them swing, but based on what you describe here goes: generally someone who slices "comes over the top" starting the down swing. This is caused by starting the down swing with the shoulders, this immediately causes the club to go out and the only way to get it back to the ball is swing from the outside across the ball. The downswing should start below the waist with a weight shift to your lead side, try this without swinging at a ball and you will find that when you do this your trail shoulder will drop down and you will be swinging from inside the ball to outside. One thing you can also do at address to help this is to take your stance then drop your trail foot back about 4-6" away from the target line (closing your stance), this is counter intuitive but trust me. This also makes it harder to come over the top. One question I do have is where is your weight at the finish of your swing? Is it on your lead side or do you feel like you're falling backwards? The reason I ask this question is you mentioned keeping your head down. This is rarely a problem. If you feel like you are falling backward at the finish of your swing, you have what is called a "reverse weight shift," and it feels like you are raising your head but you really are not. Starting the downswing by shifting your weight will also help this. Good luck.
If it isn't on his lead side we can't help him. He needs to see his pro ASAP. Your mentioning of starting the downswing with lower body cannot be over emphasized. If he isn't doing that on a consistent basis he's in deep shit. And Downy doesn't make enough toilet paper to help him.
I'll give it a shot, I am finishing okay (weight is on the front side) but it is possible the shoulders are a little more involved than they should be. I will work on it when I hit the range in the morning.
@65Grad How long have you been playing are you still playing? I played for about 50 years before I had to give it up because of lower back problems. And damn I miss it.
Hell I don't know. I'm gonna make sure I start with the hips and see if that changes anything. I should also note that it isn't an elaborate onto the next fairway slice, its actually a nice little fade but I don't know how it got there and it bugs me. I should probably leave it alone because it works really well on my course because all but one of the dog legs are made for a shot like that.
If you're hitting a gentle fade on a consistent basis with all the clubs in your bag, I wouldn't fuck with it. As Trevino once said "You can talk to a fade, but a hook won't listen." Truer words were never spoken. Also as I mentioned earlier, spend the majority of your practice time on the short game. I'm sure @65Grad will agree with me.