Hey Dano. First of all, welcome to HSBBW. My opinion is that you should definetely work on the foot landing. Many problems can wind up occuring with an open landing. One of the things I used to do with my son is go play catch on a field with a line. Didn't have to be a baseball field, a football or s****r field. After every throw, I would have him look down to see where he stepped. Get it ingrained in his muscle memory to step towards his target. Drawing a line on the pitching on the pitching mound is a good idea as well. I used this even with my 14 yr old travel pitchers.
My son also throws from a low 3/4 slot and has been successful with it. The thing you want to make sure he is not doing is coming around with his hand under the ball. In other words, if his forearm were at a 90degree angle to his upper arm, you would want a 12-6 spin on the ball. If his forearm were straight out with his elbow, you would want a 3-9 spin on the ball. Figure the angles in between. It's ok to have side spin on the ball. As he throws harder, this will give his ball movement.
Hope all this makes sense. Good luck with the pitching. It's been (and still is) a lot of fun watching my son pitch.