I think you're right that if Nana isn't worried and agrees with the doctor than there isn't much you can do except keep working with him as you have been and hope that helps.
It's kind of along the same lines as when parents don't discipline the same way we do. We can only do the best we can while they're in our care.
It's just hard to watch because its only the child that suffers. :(
The program with PECS symbols used to be called "Boardmaker" Not sure if it still goes by the same name.
From what you have said before about dcb and nana and their situation, I still think that a lot of the issues are around the whole family situation. Nana is very likely overwhelmed. The mum and dad are not really in this kid's life and she is doing this all herself. Does she work? or is he in daycare because she knows he needs it? That may make a difference too. If she isn't receptive to looking for more help, ie. other professionals, then for the short time you have him, I don't think there is anything more you can do except what you are doing and plan to do.
I was going to offer some suggestions about the food situation, but I went back and reread your previous posts. Seems like you've thought of all my "tricks" anyway. With your limited time with him, seems like you might not be able to make much headway.
Oh, and about that Doctor.... sometimes, I'd like to just.... nope, can't say it! I have had parents come back after dr visits for flus, strep, etc. and the Dr. has said to them "Oh, she's in daycare? She shouldn't be in daycare!"
The health agency around here used to do dev't accessments of all kids at 1.5 and 4.5. I've had parents come back practically in tears telling me they feel like failures because the child didn't do some stupid, random thing on the spot so the nurse says "your child might have a delay." But when there are actual issues, (like in your sit.) they say nothing. ARRGG.