Unlike surnames heraldry is displayed by physical objects in-game. The customization of coats of arms is a design experience meant to make the integration of your personal heraldry into the game seamless on day one. However, because they are physical objects there is nothing stopping anyone from wearing it, duplicating it, or stealing it if they can figure out how.

It wouldn't be that weird for the non-noble soldiers of a noble to bear his coat of arms. Likewise, a baron flying his liege's coat of arms alongside his own would not be weird either. Further, there's nothing stopping people from making new coats of arms as much as they want after launch.

In short, he can probably do whatever he likes. He will not be locked into flying his Counts CoA (that would kind of defeat the purpose of creating his own), or locked into anything else relating to this matter. They are physical objects, if he wanted to he could change his coat of arms every other week after launch, although I strongly advise against that, ha ha.