I have never claimed to be a baseball guy, but coaches eliminating more than half of their potential 1Bs based on left-handedness seems bizarre to me. My son has transitioned from C to 1B over the past couple of years. In his experience at the HS level, the priorities, in order, have been:
1a) hit for average
1b) hit for power
2) soft, sure hands
3) decision making (when to pick, block, hold the bag, knowing the park, cuts, etc.
4) footwork and quickness (not speed) on defense
5) arm strength
I think your son plays C, too. The first time he drops down and takes one off the chest to save a big run, his coach will be glad that he has the catcher background.
Too funny, he did that in a game last year while playing first he wound up in the middle of the infield and blocked the crazy throw from Center and I just rolled my eyes like "Did someone forget what position we were playing?", LOL!