I'm so sorry to hear this, and I'm surprised that they didn't blood test for FIV, which is fairly routine for un-neutered males
They certainly should have not have homed without prior warning of any of his health issues, such as teeth, so when you're feeling up to it, I would certainly speak to the rescue, and also make sure they pay any outstanding vet bills, as clearly this could have been avoided if they had been up-front with you (its certainly worth a try).
On the flip side, if Naboo had tested positve for FIV, they would almost certainly have put him to sleep there and then, and although his life with you was cut short due to this illness, he had 5 wonderful months with you, in a loving hime, which he wouldn't otherwise have had
RIP Naboo, safe and sound on the Bridge now, go find my FIV bridge boy Max xxx