He can't make proper use of his food right, as his numbers go down, that will change. Asia lost some weight too and gained it back in short order (and now being at normal weight has exasperated her arthritis, but that's another story). I'm allergic to cats as well, not as bad as some people, but if I get fur in the face, I'm an itchy, stuffy mess. Never stopped me from having cats though!
I found it helpful early on (and still do) to celebrate the good things, like you said, playing, purring, heck, I celebrate when Asia attempts to groom herself because she hasn't done in over a year! I keep it in my notes at home and try to put it on my spreadsheet. More than anything, it reminds me she is not her numbers. Slowly, some things have come back that she hasn't done in awhile and that really makes me happy. Sometimes she does those things when she is in green, yes, but sometimes she's in pinks or higher numbers. I think once they start trending down, the results are cumulative and carry them through all the bouncing and a few hours in yucky colors, or so has been my experience anyway.