Is it obligatory for me to make up the ten days of Ramadan that my deceased father missed, and is fasting them on his behalf considered an act of devotion to him?

The Prophet, peace and blessings be upon him, said: "Whoever dies while owing fasts, his guardian must fast on his behalf." It is understood from this that if a deceased person owed obligatory fasts from Ramadan (definition) or a vow, and had sufficient opportunity but did not fast, then his guardian must fast on his behalf. If he did not find an opportunity to make up the fasts, then there is no sin upon him nor is his guardian obligated to fast. If your father's outstanding days of fasting were due when he had the opportunity but did not make them up, then his children must make them up. You can divide the fasting amongst yourselves, or one of you can fast all of them. This is an act of righteousness towards your father.