okay so firstly i imagine he had very little room for self-expression as a child. his father wanted him to dress a certain way and act a certain way and that was that. he's pretty much forced into stereotypical "boy" clothes because that's what his dad wants him in and that's what his mom knows keeps his dad happy.
when he's around 14, he gets kicked out or runs away. bob plays with his personal style a little bit (this is when he first grows his hair) but quickly realizes that in order to survive he needs to make himself look older. so he styles himself older and that generally means wearing clothes he wouldn't otherwise choose for himself. thankfully at some point he does settle in enough to feel comfortable branching out of this persona.
he starts trying a few different things like goth and grunge and even some more cutesy, gender nonconforming pieces that his father would never let him touch. but he soon learns that dressing that way is dangerous. there's a fine line between feminine and masculine and when he crosses that line into feminine, he becomes the target of terrifying men (specifically those he is buying drugs from and doing drugs with) so he leans more into his masculinity and settles on a slightly off-putting grungy look (like in his mugshot) because it seems to get him the least attention.
post-thunderbolts, he starts over. new life, new bob. mostly he ends up wearing t-shirts for artists and games that he likes along with basic sweatpants but he does find some other items. crop shirts and shiny pants that lean more feminine, muscle tanks and basketball shorts that show off his masculine body. everyone seems fine with whatever he wears so he mostly sticks to what's safe but branches out for fun.
it's still hard to lean into his interest in feminine things, though. bob lets yelena do his makeup but can't bring himself to wear it outside of her room. he's bought at three shirts that he bought and then decided he wasn't bold enough to wear. his father is always in the back of his head telling him boys don't wear those things.
then he starts dating john. and he knows it's irrational but he's really fucking insecure about being a dude with john. bob knows that john loves it when he wears cutesy panties and lacy shirts, when his hair is all curly and pretty, and he gets it in his head that john is not attracted to his masculine side. so he overcorrects. he shaves twice a day, puts away the basketball shorts, tries to make himself lean feminine but beyond a certain point he's just not comfortable with it.
so bob ends up pretty much wearing all big sweaters and sweatpants because he hates it when he looks too masculine (like his father wanted) but he's not comfortable looking too feminine (like he thinks john wants) so the "safe" option is just to hide his body entirely. and maybe what he won't admit is it all has something to do with his father and with other men in his past who tried to force him to be more feminine or objectified his body and now he's just not comfortable existing in his own skin (especially after the sentry serum made him more conventionally attractive).