The FHA prohibits discrimination based on familial status, which includes restrictions that would exclude a family with people under 55. HOPA carves out an exception under the FHA permitting some discrimination, but not for children under 18 in a home where there is a 55+ member. I have litigated this issue. I'm pretty familiar with this area of law.
The only reason they can kick the kid out in this case is because he is 18 or older and doesn't fall under the HOPA protection like children under 18 do.