Would you say he has lost a worrying amount of weight though? Is his body condition still ok?

At that weight he probably is capable of breeding so might be in breeding mode.

I find my males require less food than a female of the same size and usually feed slightly smaller prey items as a consequence. I have one which goes on a winter fast every year despite that though.

I think, unless there are other bad signs/ symptoms of disease, You should just keep monitoring and offering now and again - though I would suggest trying a day old chick as a tempting treat as some of mine go daft for them.

There is also a thread by Justin Kobylka that has some intresting theories and strategy's.