People don’t become angels - ever. He’s better off as a saint in heaven than any angel will ever be as just an angel. Remember, no angel will ever get a resurrected, glorified body. That little boy-hero will.
Yes, I know, it was just a sentiment. Angels can and do intervene in this our physical existence, but the saints can only intercede to G-d for His help, aware as they are of our lives here on Earth.
I would still like to think of Xander as an archangel guarding his little sister even now. Only a sentiment.