All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The position and status of the grandfather is like that of the father, so it is an obligation to be kind and dutiful to him, and the manner in which you related the story of your grandfather suggests that you are unkind and undutiful to him.
As regards his marriage, it is permissible for an old man to marry a young woman. The Prophet married while he was aged more than fifty years with 'Aa'ishah while she was only nine years old. [Al-Bukhari and Muslim]
Therefore, if marriage took place while fulfilling the conditions of a correct marriage, like the presence of the guardian and the two witnesses, then the marriage was valid. Your grandfather is not obliged to inform his family about it, but if he did so, it would have been better.
Saying that he marries because of his desires, then this is something normal because if he does not satisfy his desires in a lawful way he would resort to do so in a forbidden manner. If he marries with the intention of keeping himself chaste, he will be rewarded for it. The Prophet said: "When a husband has sexual intercourse with his wife, he will be rewarded as if he spent charity. The companions inquired: "Are we really going to be rewarded for satisfying our desires [with our wives]?" He asked them: "Is he not sinful who satisfies his sexual desires in a forbidden way? Similarly, if one satisfies his desires in a lawful way [with his wife], he will be rewarded." [Muslim]
There is no doubt that there is much wisdom in the marriage of the Prophet with many wives, among which is what you mentioned. However, this does not mean that the fact of your grandfather loving women is not one of the wisdoms of marriage because of natural inclination. It is confirmed that the Prophet said: "The most beloved things to me in this life are women and perfume, but the comfort of my heart is in praying." [An-Nasaa'i] So this is not a dispraised matter; however, it is completely wrong to believe that the Prophet married only for this purpose, and thereby degrade and disparage him.
Again we stress that you are obliged to be kind and dutiful to your grandfather as his position and status is like that of the father. So it is not permissible to misbehave towards him or talk about him in an improper and repulsive way, let alone accusing him of cheating and deceiving without evidence. In principle the actions of Muslims are innocent, especially if they are legitimate like dyeing the hair with a colour other than black colour. How did you know that your grandfather dyed his hair to cheat and deceive people? Why did you not consider this as a Sunnah and a way of beautifying himself if he dyed his hair with a colour other than black colour?
To conclude, it is permissible for him to marry, and it could be a Sunnah or an obligation for him to marry. Therefore, it is not permissible by any means for a person to be undutiful to his father or grandfather even if they disobey Allaah. In principle, a Muslim is innocent until he is proven otherwise.
Allaah Knows best.