This University of Texas online course is free and appears to be pretty basic:

http://www.laits.utexas.edu/fi/home

I plan on using it with DD14 this summer. She was in an immersion program from Pre-K through 2nd grade, but she hasn't kept up with her French.

A tutor is also a good idea. Are there any older kids at his new school that could help? My eldest, DD16, is fluent and DD14 sometimes helps her younger sister (DD7) with her homework (DD7 is still in the immersion school).