i'm in your conscience
Nincada probably won't help much, until it can learn Mud Slap anyway. I think it learns that before it evolves, so go for that. Aron also gets Mud Slap, and Makuhita learns Arm Thrust at level 10, which is good against Magneton. Geodude is also immune to all his Pokemon, so if you're up for going back to Granite Cave, it's probably the best option you have at the moment. And Grovyle will get Leaf Blade at level 29, so that might help against his non-Steel types, plus it's a great move in general. Don't get rid of it when he does learn it, actually haha. Hope this helps!