Only thing different on this is he's flying as a non-rev from a companion ticket voucher I got with my MP Visa

That is the reason it popped up ssss on the airline, we get those all of the time for non-rev tickets. Many times the gate agent can fix that but some are lazy and won't. I always thought if the child was under 12 they ignored it. I am so looking forward to a new HS person. The SSSS stuff has been needing a major overhaul since day one.