It's never a problem to have them keep the sauce off the steak, so no worries about that. If he will eat a steak, he can eat at pretty much any of the signature restaurants--just ask for it plain. I do it for my 9yo dd all the time. You could also order a childrens meal (if there is something he likes) for one of your younger children, and perhaps they could eat something off of the adult menu (since you said they are more adventurous) so they could switch meals.
One other option of course, is to simply have him eat something else either before or after the rest of you eat.