Fan here.
No, the boys are not bullying Sunoo. Yes, Sunoo laughs along and I don't think his laughter is fake.
But it's clear that Sunoo has body image issues - always talking about how he gained weight, needs to lose weight, worried about showing a double chin in pictures, saying losing 0.2 kg made him happy recently, also these videos where he refuses to eat cake is heartbreaking.
All the other members must know this. Sunoo isn't a weakling but it's clear he's insecure about his body image/weight gain. So why would they still make jokes like "Person who weighs a lot -> Kim Sunoo"? They are idols themselves so they should understand.
I genuinely believe they care about Sunoo but idk just feeling sad and conflicted about this especially right before comeback. Sunsunki is my favorite trio too 
