Let's be realistic.

A fire hot enough to melt brass and nickel has certainly softened and
warped the frame and made the internal parts useless.

Simply polishing and replating the frame and cylinder would cost about
$200.

IMHO - you could have the cylinder welded shut , and mount it on a
nice wall plaque with a brass name and date plate as a souvenier.

If you want to "replace" his gun start saving (maybe with help from other
family members) and buy a real one for his next significant birthday.