The Reptile Wars by Jim Edmiston
Vinny Balfour crashes out of the top floor of his burning mansion on a motorbike with a dragonscale blade hidden by his crazed, scientist father, fearing the Malsaurians. Vinny is surprised to find that he’s not dead but thrown back to a time where a grief-stricken mother takes him to be her long lost son.
This other England of the 13th century is run by the weak Prince Ynyr, controlled by his protector, Phar Lazar.
Vinny finds himself caught up in a Christian crusade against the Moors of Aragon and the warlord El Drac, thought to have demonic powers. It is Vinny who finds out for himself who is at the heart of the Reptile Wars.
His outcome, of interest to another force from the future, is not assured.