Does Allah forgive the sin of one who abandoned prayer and has passed away, knowing that he was a monotheist, a Muslim, and of good character? And can anything be done for him after his death, other than supplication, to expiate his sin?

It is not permissible to seek forgiveness for someone who abandoned prayer out of denial of its obligation. As for someone who abandoned it out of negligence, while still acknowledging its obligation, many scholars do not consider him a disbeliever and permit seeking forgiveness for him and giving charity on his behalf. Some scholars believe that he disbelieves unconditionally, and according to this view, it is not permissible to seek forgiveness for him. The preponderant opinion is that there is no harm in supplicating for him, due to the possibility of him being a Muslim.