((((Deb))) you've got a really challenging situation.
My first thought is to wonder if the weight is real weight or if some part of it is fluid? Has any vet assessed that?
Next thought is metabolism - could there be something that's slowed his metabolism down so that he's not burning calories at a typical rate? I don't know much about cat metabolism issues, I'm just brainstorming.
If he didn't have the negative acro test I'd think it was definitely acro. Does he have an insatiable appetite, or is it a normal hunger?
In fact, I wonder if you might want to talk to one of the acro vet specialists and ask if they know what other conditions to look in to since his acro test was so low. I think I'd call Dr. Kathy Lunn - she spent a ton of time with me on the phone talking about acro and helping me make the decision to get punkin treated. I'd guess she is the most knowledgeable in N. America about acromegaly - she started the treatment program at Colorado State University, including running studies on acro, then moved to North Carolina to start a program there.
https://cvm.ncsu.edu/directory/lunn-katharine/ I just called her, left a message and she called me back. She's really approachable.
Earlier this evening I was just rereading
this link that Wendy found about insulin resistance in cats. Ironically it doesn't even mention iaa, which we see here pretty regularly in high dose cats. But you may want to take a look at it and see if there is anything new there, if you haven't read it yet.
Hang in there!