As far as I know, Japanese food is not as nut-centric as many Asian cuisines. I haven't researched Japanese steakhouses in a very long time, but ate at one once many years ago. I don't remember there being any nuts anywhere on the menu. Since they are cooking in front of you, everything is very clean (probably much more so than if behind closed doors), so IMO cross-contamination is less of an issue. Plus, the foods are mostly fresh, whole ingredients (meat, veggies, etc) - so less chance of being a problem. If you would feel comfortable with him eating at an American steakhouse, then I think the Japanese would not be any riskier. Do you usually ask restaurants to check ingredients of the components they use in your DS's food, or do you just go by the menu? Either way, since you're unsure about the Japanese place, you might call and ask if you can come in during a non-busy time to read ingredient labels on the sauces and other prepared items. The most likely culprit I can think of would be the oil they use when cooking - it's pretty high-temp and peanut oil is good for that.
Glad you came out of lurk mode (even if temporarily)!