Would repentance be accepted from someone who stole his father's money and cannot return it or inform him for fear of severe consequences, knowing that he has repented and become observant of his prayers after he used to not pray?

The door of repentance is open, and Allah's bounty is immense. You must hasten to repent from sins, for Allah rejoices at the repentance of His servant and forgives all sins. It is part of Allah's mercy towards His servants that good deeds efface evil deeds. As for the prayers you missed, you must make them up as a precaution. The money you took from your father without his permission must be returned by any possible means. If you do not know the exact amount, then err on the side of caution with an amount that absolves your responsibility. If you do not possess the money now, return what you can and resolve to return the rest. And frequently recite: "O Allah, I seek refuge in You from anxiety and sorrow, and I seek refuge in You from inability and laziness, and I seek refuge in You from cowardice and stinginess, and I seek refuge in You from the burden of debt and the oppression of men."