I don't know how old your DS is, but my DD (now 10) got similar feedback when she was in preK. It was a 'its really nice to see her be silly every once in a while' more than a 'she's too serious', really. And they were definitely encouraging her to silly it up.
It sounds like for your DS its not a similar scenario, but just in case I thought I'd add our experience with this. For my DD it was about letting go of 'control', definitely an aspect of perfectionism for her. She has a tough time handling her emotions (that and she's pretty intense in her feelings and reactions to her feelings, in general, whether its a worry/problem she's 'stuffed' or a newly skinned knee that she's wailing over like a broken leg ... that part used to be a lot worse, she can handle the actual pain reactions better now, but at the time of the comment there was immediate and major drama to very minor injuries). When she was younger that control would inevitably relax when she was sillier. So she just kept some of it in, especially in situations where she would be embarrassed if she lost control.
So for her, seriousness was a side effect of not allowing herself freedom, therefore it was somewhat of a concern. Make sense? While it does not sound like that is what is happening with your DS, perhaps the teacher has something more like this in mind, and is concerned he's just not having fun? (And, maybe he's not at school...?) Perhaps reach back out to the teacher, and ask for a quick follow up conversation. Sometimes clarifying what someone has said really helps -- I've definitely left conferences and meetings and realized I'm confused by something that was said, and find that for me at least, it has helped when I have followed up.
