Your son could be setting himself up for disappointment, however, IMO he definitely has a better chance as a walk-on at JuCo than at D1/2/3. For one thing, JuCos typically play more games in the fall. That will allow the coach to make a better assessment of his talent than the more restricted schedules of D1/2/3. Also, JuCo rosters are much more fluid than four year schools. Obviously they have to replace about half of the team every year, plus a few guys do get drafted or move on the D1 after one year. Just like D1/2/3, some guys will wash out academically in the fall. On the down side (for your son), players can be admitted right up to the last minute, and they can transfer in after the fall semester somewhere else and play immediately.
Is your son talking to the coaching staff now? If not, he should be. They can probably give him some idea about his chances of making the team.