Maybe his dad misinterpreted how he was acting after Maddie's phone call as stress, when it was actually more like excitement or anticipation. CS did particularly note that he didn't think he was "upset", but he seemed to not know what he should call how he was acting, but eventually he chose the word stressed. Yes, that makes me think it might have been more like excitement/anticipation.
Which would also fit with him suddenly looking for sedatives/anti-anxiety meds. In fact, SS may have been pretending to feel stressed or anxious, putting on a show for his parents' sake, in hope that they would share some meds with him or to explain why he suddenly wanted to find some meds, as CS said he did. But the real reason he wanted sedatives may have been to give to MS later.