Hello
I think it is great that you want to get your guinea pig a friend. However, before you do so, I think it is important to make sure that he is 100% healthy. If he is unwell when you try to bond him with a new boy, he may not react how he would if he was feeling well (for example, he may submissive towards him at first, but later try to assert himself more when he is fully recovered).
Can I ask you to explain his health concerns in a little more detail? You say he has stopped eating his main food, but that he is still eating things like carrots (which is good). When you say main food, what exactly do you mean? Do you mean hay and nuggets?
I'm also a little confused by the weights you quoted - he went from 0.94 to 0.90 in about 3 weeks - do you mean 0.94 kilograms (kg) and 0.90 kilograms? If so, this this is 940 grams to 900 grams, which is not a very big weight loss - I believe a big wee can account for 30 grams... however, you may have meant something else?