By the time he turns 18, he will have played tennis for almost 5 years.
Most players peak after 3-4 years, get bored, lose interest, gain interest in other things...girls, motocross, surfing....
5 years is plenty to make 5.0 level, or higher if he's tall and quick.
Div1 is mostly 5-5.5, with a few 6.0 sprinkled in.
San Francisco has a history of players starting late in life, and making the top levels. CeciMartinez, Marcie and MareenLouie, make pros starting around 13.