If it's 400 feet in 1.5 miles that's a 5% grade. To maintain a 70 RPM pace with a 32x34 I'd be going about 5.3 mph. At my current weight (200 lbs) I'd need to maintain 138 watts. Adjust accordingly for your son's weight.
EDIT: At 120 lbs you only need 90 Watts.
I don't think a 5% grade is too steep to need to adjust the bike.
Last edited by guachi; 06-18-19 at 11:17 AM.