LOL, I find this advice ridiculous. That's complete nonsense, to put it lightly. Many middle aged men in Bulgaria not only wear shorts in summer but they often go out with shorts ONLY - this means they are topless (the fact that they have a fat and hairy tummy doesn't seem to worry them in the least). Of course, they don't go to work looking this way.
I really can't understand where the advice about white shoes comes from? For 24 years living in Bulgaria I have ever heard the rule not to wear white shoes.  Maybe they are not the best choice because they get dirty too easily but omg, nobody will find it strange. Only men in pink may look strange but nothing bad will happen to a man wearing pink, actually many Bulgarian men do wear pink these days. 
Well, if your father wears a burqa or a skirt, that will definitely look strange but something tells me he is going to wear any of those anyway. Anything else is fine, really. Actually, I don't think he will face any problems even he's wearing a skirt but I would still advise against wearing a burqa. And if it's over 30 degrees, well, I honestly think wearing shorts is not only normal but necessary.
About the church thing - yes, officially you should be fully dressed to enter a church but I and all my friends have visited many churches and monasteries wearing shorts and sleeveless tops so nobody is too strict about it. All Bulgarian girls wear skimpy clothes, please don't worry about them. Yes, grandmothers are always concerned about you getting a cold but that's just in their nature - young people don't care about it too much. So any girl wondering whether it's ok to wear skimpy clothes - yes, it is (but if you are too much out of shape some people may comment behind your back about fat foreigners - but still, most people just won't care and almost nobody will judge you).
As a tall, blonde man your dad may receive some more attention as a foreigner but not that much since he is Caucasion and there are blonde Bulgarians as well. Also, most of the attention he will get will be positive and people might be curious about him (especially if you go to a small town). And trust me, I know what it feels like to stand out since I'm living in Asia right now. Most of the time I enjoy the attention, it can be really funny. It's important to have this kind of mindset and not to walk on eggshells and wonder what to do all the time. Just relax, try to get to know the locals and let them get to know you. 
Oh, I just saw that the other Bulgarian also started her comment wil LOL - this must mean something. Please, anybody planning to visit Bulgaria, do not worry about your clothes as long as they are clean and don't stink. Choose your clothing according to the weather and your personal style. Also, don't be so afraid to look like a foreigner - you are a foreigner and it's ok to look different.