skinny guinea pig

Question
My mom asked someone else, but I want to ask you about my guinea pig Junior (boy).  He is getting skinny.  He acts healthy and happy, eats, drinks, and poops - he just looks bony on his back.  We feed him green guinea pig food, apples, carrots, broccoli, lettuce, grapefruit, oranges, timothy hay.  He eats lots.  We put vitamin drops in his water.  He also has these big poops sometimes that are large, round, soft, and stinky.  My mom has had to help him get them out sometimes over the toilet.  What do you think is wrong????  Thank You

Answer
It sounds like he is having constipation problems and the weightloss is very weird. I would personally take your guinea pig to a vet because something is certainly not right with him eating so much and losing weight. Unless, your guinea pig is old, it's normal for them to lose weight but he should not be boney. He may have some sort of internal parasite or something.
Good luck and keep me posted.