The x-factor is Shawn loves being as small as
possible, there's no way he would willingly put on weight (muscle). He
spent his whole life wanting to be slender. Plus, it's much much harder
to build muscle than lose the weight. Risk of injury. His lack of coordination and
interest in fitness. Plus his low calorie & low nutrient diet would
make muscle growth impossible. He would have to change his frozen microwavable diet. He also likes being "boy-ish", he's never wanted to be a Bruce Willis type. He's described that look as "not for him" because of his face.
There's no way he can get rid of it without surgery. Building some muscle will definitely look better combined with excess skin removal, but without surgical intervention there won't be any tightening of his skin.
I've only seen his upper part of his chest, but Shawn has less excess skin than most people who've lost 200+ pounds. He would probably get good results with surgery, but it would cost him 30k+ because he would get both arms, chest and most likely tummy. It's also a somewhat risky surgery, so best not go with the cheapest option. When he bought his car could have been a time to do it, but it is an invasive surgery so I understand the hesitance.
obese_to_beast on IG is an example of someone with excess skin that combined weight loss with bodybuilding.