IB Chinese A SL vs NSS Chinese

Background
My son is studying in F3 (NSS curriculum) at St Stephen College and will need to decide within the next 2 months whether he will be taking the NSS or IB route next year.

Our major concern is his Chinese ability. English has been his major medium of learning and he is relatively handicapped in Chinese (bottom 25% among all F3 students). Unfortunately the school will not be providing Chinese B courses, so the only choices of an IB student will be Chinese A SL or HL.

Questions:

Can anyone share their experience with regard to the difficulty in studying Chinese A SL vs NSS Chinese We have obtained some IB past papers but have no clue (no marking scheme) regarding how to answer those papers and how will it be graded.

Do you think a student with the said background can handle Chinese A SL and what grade should be expected?

I know this is not an easy question? Please share whatever view you have.