I recommend a combination of opinions here: For the next 2 weeks attempt to target feed him using tongs or, as NIC stated, using a large siphon tube. If this is not working you must provide him some isolation from the more aggressive feeders. Divide the tank or move him out, fatten him up then consider returning him to your main system.
Respectfully,
FireMedic.
Respectfully,
FireMedic.