We were using Alamycin or Pen&Strep but the vet sold us Hexasol for pneumonia this year.
I know they've had ivermectin but...
We had lamb carcases with lungworm this year, for the first time, and it was nearly half of the batch too. Vet said generally we don't worm specifically for lungworm as the efficacy of the drugs is very shortlived against it (two or three weeks, the vet said), but that if in this horrible year it had got more of a hold on them, then it wouldn't hurt to give them an extra worming dose to clear them out and give them a breather - literally

. We gave them chelated minerals too, as a general tonic. And that batch did get on and grow, whereas some of our neighbours, who didn't worm again on the basis of negative FECs, have continued to complain of poor growth rates.
So I don't know whether you might suspect lungworm in your boy? It would certainly exacerbate any other respiratory problems he might have.