因为 two or more reasons. and difference with 是因为
do i have to put one for feeling weak and another for the other clause for not concentrating in his studies?.


No, I think a coordinating conjunction is fine enough, same as you did in your English translation:

因为他自己觉得身体很弱，（并且 / 而且 / 且 / 还有）学习时思想不能集中



  Could I use the clause 是因为 instead of 因为? are both correct? is there any difference? When is it better to use each one?


Yes you could use 是因为 here, and it would be correct, and yes there would be a difference, not in meaning but in what part of the sentence you are stressing.

By saying 他去看医生，是因为 you are implying that the focus of the sentence is going to the doctor, and that this requires an explanation. Compare with English: "Why does he go to the doctor? It's because he feels weak".

Whereas by saying 因为 you are implying that the focus of the sentence is the cause itself, which results in going to the doctor. Compare with English: "He feels weak, so he goes see the doctor".

As an additional remark, according to Chinese topic-comment grammar rules, this latter case where you put emphasis on the cause would require to invert the sentences:


  因为他自己觉得身体很弱，（所以）他去看医生