@Cyclemom it depends on what your definition of affordable is. With eBikes specifically, you get what you pay for. The current sway of the industry is to go for a Mid drive or Center drive motor. Currently Bosch is the most popular, however, Bosch bikes start at a price point of about $3500+
Haibike is releasing a lower price point eMountain Performance bike with the SDURO badge that uses a Yamaha Drive system.
For servicing, center drive motors are fairly easy when it comes to standard bike parts, however, the drive system is meant to only be serviced by an authorized service center as most motors connect directly to diagnostic software and with Bosch warranties are remedied with plug and play (they send the shop a new piece to replace the old piece).
The IZIP/Raleigh 2015 Models are selling at a great price now due to end of model year and have a couple of models including the E3 Peak and Tekoa iE.
At the end of the day... TEST RIDE, TEST RIDE, TEST RIDE irregardless of which brand you go with, make sure the bike feels right to your son.