How can i fatten up my guinea pig?

I know i have asked this before... probably a while ago, but. My guinea pig has been at my Dad's house and he has forgotten to feed him a lot. I now feed him every day. He gets one full bowl of pellets every day, 1/2 cup to 1 cup of veggies everyday. So far he has tried, baby carrots, grape tomatoes, romaine lettuce, and kale and has liked them all. The day after i got him, which was may 16th, i weighed him for the first time and he was 2 pounds exactly. I thought that this was too skinny because i can feel his backbone sticking out when i pet him or touch him. I started the diet mentioned before as soon as i found out what to feed him and how much and that was about 2 days after i got him. I decided that he would gain weight after a while and so i weighed him on may 22nd and he only weighed 2 pounds and one ounce. I figured to wait longer so i weighed him today (june 6th) and still, he only weighs 2 pounds and one ounce. I have herd that many peoples guinea pigs weigh around 2 pounds and 3 somewhat ounces. Almost 3 pounds maybe. I still feel his backbone sticking up. In the past he has not been well taken care of because we first found him in someones front yard near the road and my dads friend took him and brought him to us. His nails were extremely long and he was starving. We had someone at least trim his nails a little and then we got him guinea pig food right away and then my dad as i said forgot about him sometimes. So now at my house, he is fed, cleaned every day and he got his nails cut short. I have been researching a lot about guinea pigs and am trying to get him healthy again, i am first starting off by getting him well fed, a bigger cage, and i am working on getting a guinea pig cage. I am soso sorry that this is so long and i would appreciate if you could tell me anything about them but specifically how to get him some weight and what store (please not online) where i could get a big enough cage for one guinea pig but no more than about 70-75$.( I would like it to be a little cheaper than that though) Thank you, i appreciate any help you could give me :) and i again apologize for this being so long but i just don't want my guinea pig to get sick and i thought that it would help to give a little background info. :)