I would recommend using -5 if he can make the transition now. -8 at the most. Most organizations require -5 at 14u, so the sooner you can make the transition, the better. I think you'll find the majority of 13u (at least at the major level) swinging -5 at this point.
That said, not all bats are equal with respect to balance and feel. Some are end loaded, others more balanced. I recommend that you go to a store that has a swing speed monitor available and swing 3 or 4 different brand/models and find the one that feels best and provides the highest swing speed for your son. Don't pick a bat based on the paint job.
Try the Marucchi CAT as part of the group you test swing with. It is very well balanced and seems to help the kids making a transition to a heavier bat. Eastons seem to be very end loaded and harder to get around but the bigger, stronger kids get good results. Demarinis are good middle ground