2019Son went through this over the last 12 months. My observations are in line with most of the other posters, but I think there have been some positives not mentioned yet.
First, BBCOR is a huge, gigantic difference (from 2002 - 2011 there was a -3 requirement, but it was with the BESR standard, which was more similar to the 1.15 you see today with -5, -10 bats, etc.). With a 1.15 bat the ball will go ~15% farther than with BBCOR. That 300 ft. home run in 13U can be a 260 ft. fly out in 14U. 2019Son's team's home field is 330' down the lines, 380' to center (HS Varsity field) . . . I have seen 8th graders hit the warning track, but haven't seen any over-the-fence home runs when playing on that size field.
OTOH, 2019Son's coaches changed his swing this year. In 12U and 13U he had been a leadoff hitter -- get on base and steal his way around -- and his coaches this year (different coaches) had him ditch the "hit the ball on the ground" approach. I think the quote from his coach was "you should be trying to hit triples" (note: size of field makes home runs infeasible). The swing changes took some time, but net, net he has hit for more power this year than he did last year. So my takeaway is, with good coaching kids can adapt quickly.
Second, yes, as other have noted, slow kids are exposed (on both offense and defense), kids who can't throw are exposed.
Third, on the positive side, pitchers who have good movement on the ball (e.g., two-seamer with good armside run) are benefited being at 60 ft. vs. 50 ft. (or even 54). Same thing with sharp breaking balls. Plus the hitters are all using BBCOR (which is an awesome rule when your son is pitching!) For 2019Son, he has been more effective pitching (e.g., higher K rate) on the big field.
Overall, for most kids it will take some getting used to, but in a few months time it will seem completely normal. CaCO3Girl, your son will go back and see a small field 6 or 12 months from now and think "How did we play on such a tiny field?!?"