Perhaps the foster parent would let you bring Linus to their house for a 30 minute or so visit to see if he behaves differently there with you than he does in your own house. You know, like sit and pet Linus below the head to see if he bites or grabs you with the foster parent observing. Perhaps they could give you some tips. If Linus behaves like a little angel, then he may be simply still be in the process of adjusting to your home or reacting to Mille's scent.