If he knows that the company is going to take a riba-based loan, when should he sell his shares?

No File Exists
Check below answers in case you are looking for other related questions:
If a doctor does an abortion, does he have to pay diyah (blood money) and offer expiation?
Abortion of physically deformed foetus

No File Exists
Related Answers:
Ruling on short-term mudaarabah (buying and selling) in mixed and haraam shares
Ruling on opening a savings account in Dar as-Safa’, Morocco, which belongs to a riba-based bank
Ruling on investing in Internet cafes
If a doctor does an abortion, does he have to pay diyah (blood money) and offer expiation?
Abortion of physically deformed foetus

Recommended answers for you:
Performing Another ‘Umrah While in Makkah
Circumcision: how it is done and the rulings on it
The seven sins that doom a person to Hell
Differences between semen (maniy) and urethral fluid (madhiy)
Things that make ghusl obligatory