Really itd just be a guess over the internet as to the extent it is his pecs or just slight numbness, I mean there really is no way to really know what he exactly is complaining of. I mean if he went to a specialist that examined him then and took a proper history and such and did tests to rule things out then he would have a better answer. I just related as to what you were saying and your theory, then tried to explain some reasons why we toss and turn and such.
His question I really dont have enough info and its just conjecture really. If it was his pec, then youd have to ask 'is it on the same side that you are lying on?' Does it only affect one side or is bilateral numbness, questions to pin it down more would affect any opinion. Really cant answer it. The old saying in medicine applies though, '....if you hear hoofbeats, think horses not zebras' so thats why I just suspect its a circulation issue from lack of tossing and turning as he normally should. The problem with that saying is of course the fact that there are plenty of zebras that can be had in medicine and thats why proper medical care is needed to asess issues.
I dont work in sleep though so I would have to defer to Juicedmohawk on more indepth sleep talk, I just know from cardiovascular standpoint, the field, I work in, that vascular issues are affected by gravity and movement.