Definitely contribute the max this year and next year to your own IRA, whatever you decide.
What percentage would be going in his IRA? I mean, if I inherited 100,000 and we stashed 5500 into his IRA and 5500 into mine, that's a drop in the bucket and financially makes sense to shield as much as possible from taxes. After all, you pay taxes as a unit, and we always fill both IRAs even though we are single income right now. I would actually suggest that to my spouse if it were my inheritance. It's not exactly him taking it to spend on like a car or something just for himself.
But if you feel weird about it, then don't. Maybe you can tell him you'd rather keep it as one large sum in ____ (savings, taxable investment, etc) because _____ (specific financial goal, that's what mother would have wanted, it's for the kids, etc). If you start dicing it up, a little here, a little there, it does become easier to spend and may disappear quicker than you'd like. Or maybe you can keep it as savings in your name, but tell him you two can now increase his IRA contributions from his regular income this year because you can decrease your savings allocations for a while?