Think a little more long term- even though college doesn't teach you how to mow or build a retaining wall, there is still a wealth of valuable information he can learn, in both business matters and from a scientific standpoint. 10 years down the road, is he still going to be mowing? Or will he make the jump to managing, whether that be in his company or another company? Just because you can't see a direct benefit right off the bat doesn't mean you should dismiss the idea. What if in 5 years he decides to explore other options? A degree in Turf opens up opportunities in fertilizer companies, seed companies, sales, or other related industries. If he has the desire to advance his education, why handcuff him because it doesn't have immediate, tangible benefits?
I'm getting my Master in Horticulture and Turf Management from Texas Tech right now. I got my undergrad in Accounting and am eligible to sit for the CPA exam if I wanted to. Now do any classes I'm taking right now help me sit on a mower or run a weedeater? Not one bit. But my undergrad gave me knowledge of how I can utilize depreciation to reduce my taxes, learn from other companies trial and error on how best to manage processes, and the list goes on.
He will also gain a more in depth knowledge of why we do things in our industry. How do micronutrients in the soil interact with different textures and how does that affect the way plants adsorb them? What characteristics of a plant's physiology make certain herbicides more effective than others? It will never be a bad thing to have more knowledge and understanding of what you are mowing.
Honestly, we need more educated professionals in the industry, and if he is college age right now, this is the perfect opportunity to gain life experience and mature while gaining valuable knowledge to make him a better landscape professional.