First off, i'd make sure he really wants a dog, and the responsibilities of it. Maybe you've already done that, but it's important if you haven't.A good idea would be to go to your local animal shelter, and tell them exactly what you are looking for. You might find the perfect dog for him. While it may not be a purebred, or a pug, many mixed breeds make great dogs. One of the advantages to this is that you can often get a dog that already has an amount of training and socialization, maybe young, but already housebroken for example.Of course you can watch your local newspaper, and see if something turns up there. People sometimes need to find a good home for a dog they just can't keep.As far as breeds, and guarding, my favorite breed has always been the Bull Terrier, although this might be a little larger then you are wanting. There are other terriers I like also; however they often have somewhat stubborn temperaments.If your father doesn't mind long-haired breeds, and the care involved in keeping them clean and un-matted, that will broaden your possibilities.Since a purebred dog may be outside your budget, I'd suggest checking out the more common breeds, to find out what characteristics they have. Then you'll know what to seek out or keep your eyes open for.A good site for this is IAMS site, where they have a lot of good and well-organized information on each breed:
http://us.iams.com will take you to their home page.I hope I've been some help. Best of luck to you. ~smiles~