Among other reasons, such as safety or practicality, there is also the issue of the value of business class. If the airline allows one upgrade to support two people, then there is less incentive for people to buy business fare or buy upgrade. Don't forget that good sleep is a big part of why people pay for business class.

From what I understand, it is generally OK to go down to the lower cabin, but not OK to go up to the higher cabin. If you can, upgrade him too, so you two can both sit in business class and there is no concern on what is proper.