My dog is a 'bad' eater. Instead of him gulping down his daily ration, he gets it in the AM and noshes on it all day like cats often do.
He has been boarded when I travel at the same kennel for nearly six years. He LOVES the management there (to the point that when I drop him off he goes "whoo-hoo!" and jumps all over the place)
Despite that, he still seems to alternate days when he eats with days when he either nibbles here and there or simply refuses food entirely.
Buster's staying with you is a huge change in pack dynamics and it is actually NORMAL for many dogs to not eat or not eat well under these circumstances.
I wouldn't be at all worried so long as he is excreting and otherwise behaving normally.
The microwaving the wet food serves to melt any gelatin in the can and it releases more of the aroma.
My cat won't touch cold wet food and if I give him actual meat I have to warm it up to 'blood temperature' before he'll eat it. At the same time, Mr. Picky won't touch it if it is actually cooked.