I think the IRS tool should work great for you on your federal withholding. It's certainly possible he's having more withheld than necessary, and that tool will help you determine where you're projected to land. If it looks like you're going to get a substantial refund, feel free to use the slide bar at the end of the IRS tool to adjust your end results to where you want them, and then click the "How to Adjust Your Withholding" to get details on exactly how to update the W-4(s).
As for the states, that can get tricky! Especially the withholding part. For what it's worth, it sounds like you're still probably best off filing married filing jointly for both states, as Ohio allows you to each have different residency statuses even on a married filing jointly return, and Montana won't tax nonresident military earnings. Then you also won't have to consider which of you is claiming which dependent(s). As you think through your withholding amounts for each state, keep in mind what each state will and won't tax - it should at least help.
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