My thoughts are this. Horticultural degrees etc are useful...but Business Mgmt is something he can use for example in the business if he still loves it when he gets out of school and use for someone else if he does not still want to do your line of work.
There is a reason that many start ups occur in this industry every year......there are also many reason that most of them fail. Some of them could of avoided failure if there was a solid business background to support them, often starting out with a business plan.
I think having a in depth knowledge of business is under rated in this field. Many of the day to day things he can learn in the field with you maybe while working his way through school and come out of college with the knowledge of both?