BS”D
The problem is not how he is dressed.
It is that he clearly KNOWS he should be dressed differently, FLAUNTS his disregard for the rules and looks for validation here.
IF it were an informal shul, well, I would say yes, Hashem is very happy to see people learning dressed for boating or golfing in such a shul – because perhaps otherwise they’d have passed by the reform tembel on their way to the marina or golf course.
But this is clearly a situation where the original poster knows he is acting with disrespect for anything other than what HE feels comfortable in, and how HE feels comfortable observing HIS version of Shabbos which is a pale copy of the real thing.
As for OTD being caused by dress rules – nonsense. And from the many MO I know who went off the derech the moment I got to college, there is as big a problem with OTD in the MO world as there is anywhere else. The difference is that the MO who go off do so because they don’t have the weapons to combat the kefira they are taught in college, and they are easily swayed by it. Besides, the dip from jeans and a polo shirt with a knitted yarmulke to same without a yarmulke does not seem like a big deal for some of these MO dropouts who had one and a half feet planted in the secular world anyway.