Hello Alexa, I have been to all of those countries and could give you a great deal of advice.
Generally, as I've aged (50), I have subscribed more and more to a "less is more" approach and thus if I was approaching your trip and "Romania is a must", then I would just limit 4-5 weeks to Romania and one other country (I would choose Bulgaria myself or Macedonia or Montenegro from your list).
Now, when I was in my 20s, 30s and some of my 40s, I, too, would have tried to (as efficiently as possible, of course) cram as much as possible into what seems like a long time. Therefore, it depends on how you're feeling. If you and your son want to do a younger-person's Rick Steves-type trip, you could see nearly all of these places since the territory of former Yugoslavia, encompassing Croatia, Serbia, Montenegro, Macedonia, is relatively small. In fact, Romania is larger than all four of those countries combined.
If you wish to let me know some of your interests and what pace you think you might prefer, I'd by happy to chime in with six or seven chapters of tips. Also, does it have to be August? July is a lot better, though June and September are much, much better than either July or August. Your son's school is an issue, no doubt.