I think you may be mistaking his "guilty" look for one of submission. Ears folded along the head, head lowered and slightly wagging tail usually shows a submissive dog greeting a higher ranking dog. Basically the polite doggy way of saying "Hi Mum / Dad, I missed you".
You could always try to teach him to bark on command with a command such as "guard" or "protect". It wouldn't make him aggressive but can be scary for someone who doesn't know your dog. (I have heard of this successfully working with a Labrador.) The owner taught her lab to bark and bark and bark when she heard the word "Protect" and then when she says "Watch" the lab would stop barking. So anyone hearing that would think that the dog was still ready to 'protect' their owner.
There is very little you can do to make a dog bark at strangers, most of the time they either do it or they don't (unless your training proper guard / attack dogs.) Getting another dog who barks may work as you said he barks with your parents dogs. It sounds as if he is quite unsure of himself and not really prepared to draw attention to himself by barking when he has no one to back him up, so having a mate around might bring out the guardian in him.
How he will get on with another dog is all about him and the other dog. Some dogs get along and some don't, just like people. I would suggest thinking about rescuing a dog from a shelter as you and Bear can meet the prospective new family member together and make a decision (i.e you like him, Bear likes him). What sort of dogs do your parents have? There is a fair chance that another dog will liven him up as this is happening with your parents dogs, but then again it is down to the dogs. Especially if you get another dog who doesn't like barking very much
"In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog." - Edward Hoagland