We managed to do a test ride. Set Google Maps as follows:
Starting: Tenafly High School
Destination 1: Starbucks (803 W 181st)
Destination 2: Tread Bike Shop (250 Dyckman St)
Destination 3: Hastings-on-Hudson, NY
So we avoided the dreadful stairs.
Nevertheless, going through Van Courtland (after some rain) was kind of tricky cause of the Old Putnam Trail/South County Trail. However, it is probably the best choice cause safety is key and I want him to avoid traffic to maximum extent. Bronx taxi drivers are horrible and make U-turns with no consideration for law.