Hang onto the electronics and motor.

You can purchase ARF profile foamy planes that are a blast to fly, don't require much work to put together and will accept the servos and components.

However these are for more advanced fliers.

You may want to take your step son out to a Club meeting, and see when the club provides training sessions, etc.

This is ultimately a cheaper route to take, as more formal training will help prevent those expensive crashes.

The ultimate goal of club training sessions is to make the pilot proficient, avoiding damage to planes and property, and keeping them interested in the hobby.

This may cost a bit more up front for a good trainer, etc. but it is actually cheaper in the long run than the alternative of frequently crashed planes while learning.