When my kids were that age I started riding “MTB” with them. They rode simple MTBs and I rode my single speed road bike on the trails so that I would be as challenged as they were. After two or three years of that we got nice new bikes and started riding more & more. Make sure your son is having fun and then you can ride together for years. It can be such a fantastic experience. I’m super happy with my 23 lb hardtail and 2.4” tires.