My guinea pig isn't eating and seems depressed:(?

Okay I know it's early and it's probably just him getting use to his new home, but this is how the story goes.. I was in petco and ended up buying a guinea pig which I named Pig. If I had done the research earlier, I would have gone to a rescue. Anyways they couldn't give me an age..they said their guinea pigs are less than 6 months old....they sold me a 70 dollar cage (which wasn't very big) and pellets called fiesta mix. I came home, and a few days later, I started doing more research and found out I should just have plain pellets rather than all of that dried fruit and colors in it..and that his cage had to be bigger and he needed constant hay...I started working a lot more so I bought another piggie. I got a baby boy and he was from someone I knew whose guinea had babies.... The one from petco already is use to me..he sleeps on me, and eats from my hands...HE WILL NOT eat any lettuce though...only bell peppers...the baby from my friend whose pig had babies loves lettuce..everything i give him..like cucumbers, lettuce, kale, cilantro, parsley....Pig ( which is the one from petco) will only eat that fiesta mix...I've slowly been changing it with better pellet food , but all he does is dig into the dish to find parts of the old fiesta mix..he wont eat any of the lettuce i give him or any of his vegetables..and the other one ends up eating it...there have been a few times where Pig watched the other one eat greens..he went up to it, sniffed it and nibbled on it but then just went to his hay or pellet food. It made me feel like he's depressed or something...Anyways, the baby boys get along great..I bought those cubes from target, and made them a C&C cage today which they loved...both popcorned for a while! even the one that hasn't been eating greens..and it made me happy..I just want the best for them..I've seriously done so much to make them feel happy..I;m trying to do everything right..I've been giving them plenty of floor time and attention...Pig falls asleep on me all the time, I have plenty of fresh food for them daily, ( which only Boba eats-_-) , they have chew toys in there and a home, fresh water, endless amounts of hay, why isn't he eating his veggies? is it just because he grew up in a petstore and they didn't have all pellet food or greens? this is my first time with guinea pigs, so I appreciate any answers. Sorry for this being lengthy.