As much as I hate to admit, I think Mario needs a friend.

[Far North]

There are a few other options to consider. Is it possible to make some puppy playdates? That certainly helped Bianca in her puppy years. I also enrolled her in part-time Muttessori, a doggie day care where they had video cams so I could watch her interactions. Also I took her to part 1 and 2 of obedience school where she also had interaction time with the other pups and found some playmates she enjoyed.
Hope you find a good solution for both you and Mario.

[CharityHand]

I don't know how long you've had Mario, but I'm with Chris on this one. I'd wait a little longer before thinking about a 2nd dog. Sometimes they just need to learn how much they're loved... and that can take some time. On the other hand, if Mario was really happy when your parents visited with their dog, then it might be a serious consideration (I probably haven't helped much at all have I?) Just keep loving Mario and I bet eventually he'll be totally happy as the alpha dog in the house.

We've had Mario for over three years so I think he should be pretty settled by now.

The play dates is a good idea, but I have a hard enough time arranging play dates for my kids!

I think for now, I just need to make sure they kids are playing with him enough and that I'm petting him enough. It's crazy around here and I think he tends to get lost in the shuffle of getting everyone off to school, work and breaking up kid fights.