even i was a science student ...did my 12the and jumped into CA wagon..there are many CAs who have come from science field ..
Since he is from a science field ..he will just have to learn basics of accounts..from some private tution...rest of subjects for Bcom are a piece of cake..
but for ca exam it will take a lot of hardwork ...(but that also applies to commerce students..)
So in end, he will be on an equal footing with all other students for the CA exam..
he has to adapt and adjust to the commerce field ..since for 2 years he did not have any subjects like economics, banking, law, etc.
you can help him here since u yourself have commerce background
I know quite a few students at 10th standard feel commerce is a field where only the average students go ( personally i have a fair share of experience about this, main reason = peer pressure)
For me, pursuing CA was the best choice i ever made after my 12th (i even had got an engineering admission in the college and was in a big delimma)
All the best