If Whistler is eating the same hay he has been I'd go with just increasing it a bit (which has already been done) and see how he does with the bump up, and I do agree - he looks great but would be nice to see another 25 - 50lbs on him right now.

Other things to consider - when were his teeth last done?  Any indication he's having trouble chewing?  Sometimes we get so focused on the IR/PPID we forget there can be other reasons that horses drop weight.  As far as the heat, even if they don't show it extreme heat can be hard on them so if he started dropping weight when it got really hot it could be related.  How has his water consumption been since it got hot?