DE may be of some help in controlling worms, but likely questonable used as the only line of defence.
Wazine is likely the most widly used poultry wormer, very easy to get and use, few reported side effects that I could find. Other broad spectum livestock wormers can be used and often are used...
About a month ago he weighed about l0lbs and I would say he feels more like 4 or 5lbs.
Have you wormed? Might be a thought, I have seen that exact situation with chickens.
Interesting artical on blackhead that covers some of the trasmission and symptoms...