We heard this too. Coaches my DC had contact with were definitely interested in multi-sport athletes, but the fact is, at the WCAC-school my DC attended playing 2 sports was extremely tough. The level of play was too high and there wasn't enough time in the day to dedicate to 2 sports evenly (particularly if the seasons were consecutive). Playing a sport like football (at DC's school) is a year-round commitment, just as lacrosse is. Sounds snobby I know, but maybe if DC were in a different conference it would have been possible.
Former IAC parent here whose son recently graduated from college and played in the Ivy League.
In today's landscape, and given year round commitments, it's becoming harder to find HS athletes who play 2 or even 3 sports in HS. Finding a two sport athlete in HS is even tough these days.
The WCAC schools, given these schools are a bit more sports focused (and perceived to have less strenuous academics) and the sheer size of their enrollments, its extremely difficult to find kids who excel / play two sports. I.e DeMatha football and SJC football are basically year round commitments.
If your son has a passion for a 2nd sport, and doesn't want to give it up, I'd strongly consider looking at the schools in the IAC. Yes a bit more pricey in tuition (most of them do offer FA packages), but much smaller in size. At our son's school, most kids in HS, played a 2nd sport as a means to stay in shape, provide a balance from the pressures of their primary sport. My son played lacrosse and football in the IAC and believe me, he had no football aspirations but he loved the competition.