It is a concern regarding SIDS, I think primarily in that it takes up to 72 hours after smoking before the carbon monoxide level returns to the level of a non-smoker. Nicotine, which is a pesticide (the only fact that actually made me quit, I could not get past the fact that I was addicted to a pesticide, it sounds so horrible and much less normal than being addicted to smoking) also has a half life, so yes, he can smoke outside and have a smoking jacket and wash his hands/brush teeth endlessly but he'll be breathing out carbon monoxide all night in higher levels than you will. I think it takes 90 minutes for nicotine to be broken down, could be wrong, but again... There's an element of being a walking cigarette. Ish.
That's what stops me taking it up again, and I have desperately wanted a lovely rollie every day for the last 15 months. But I wanted/needed to quit. So I researched it and researched it until I found my own hurdle - the bit about nicotine being a pesticide. I needed to find something I couldn't explain away.
If your partner doesn't deep down want to quit, he won't/can't.
Also one other thing to remember is that for some smokers, it's easy to quit, for some, it's relatively hard, and for some, it's mindblowingly difficult. It's one of the most insidious addictions because it's still normalised, but it's a big deal. For a lot of people, it's not something you just give up easily.