Is it permissible to sell the computer that the father bought for his son for the purpose of learning, and then buy a similar device without informing the father?

If your father transferred ownership of the device to you, then it has become your property, and you are permitted to dispose of it without informing him or seeking his permission. Even if he stipulated that you should not dispose of it, such a condition is invalid and not binding. However, if he did not transfer its ownership to you, but rather gave it for the purpose of learning, then it is a borrowed item ('ariya), and it is not permissible to sell it. Nonetheless, it is ethically appropriate to seek your father's permission before disposing of it, out of dutifulness to him and seeking his pleasure.