Then you should be fine training him in outside skills.
In real life, small forwards are almost always taller than 6'4, and there are many guards that are 6'4 or taller as well. (For guards: Dwayne Wade is 6'4, Kobe Bryant is 6'6, Jason Kidd is 6'4, Tyreke Evans is 6'6, Manu Ginobli is 6'6, Kevin Martin is 6'7, etc.)
Normally, I would want my SF to be taller than 6'4, but if he already has excellent inside skills, he would make a good SF trainee. Otherwise, he could become a guard.