Both the Mach3 and Guard are great razors but after having used both I would recommend the guard. You have to get past it's plastic feel but it shaves really well and lasts quite a while!
Why not let him try a Schick Injector. The learning curve is minimal, it gives a smooth shave, and you can pick one up inexpensively on Ebay.
Clayton
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The Guard is a great place to start. You can get a decent shave with it under just about any conditions, plus it's explicitly designed to reduce the risk of nicks and cuts, so it'll make a great starter razor. He can get fancy later if he so chooses.
The Guards arrived today, not shown them to my son yet but my first impressions of them are very good. Obviously made in such a way as to save on manufacturing costs but still very good quality razors, I was very impressed. Gave one to my other half and she tried it this morning and was also very happy with them, there's a hole in the handle for hanging them up so I can finally get rid of all her Bic disposables cluttering up the shower.