He gave his infant grandson a car as a gift, but he did not give anything to his other grandchildren

Praise be to Allah.

It is prescribed for the grandfather to be equitable towards his grandchildren when giving gifts, just as it is prescribed for him to be equitable towards his children. 

An-Nawawi (may Allah have mercy on him) said: If the mother gives a gift to her children, then like the father she should treat them equitably in all the matters we have mentioned; the same also applies to the grandfather and grandmother. End quote. 

Rawdat at-Taalibeen, 5/379 

Al-Haytami (may Allah have mercy on him) said in Tuhfat al-Muhtaaj, 6/307 

It is Sunnah for the father (and grandfather, no matter how far the line of ascent reaches), to be equitable in giving to his children (i.e., his descendants, no matter how far the line of descent reaches) and his grandchildren. This has been stated by many scholars. However, other scholars said that this equity is required only in the case of one’s own children, not grandchildren,  whether that giving is a gift, a present, charity or a waqf (endowment) or something else. If he is not equitable, without any excuse, that is makrooh according to the majority of scholars, and some said that it is haraam. The basis for that is the report narrated by al-Bukhaari: “Fear Allah and treat your children equitably.” End quote. 

Al-Mirdaawi (may Allah have mercy on him) said in al-Insaaf (7/137): 

It may be that his words about giving to children also include grandchildren; this is strengthened by the words “division among them should be in accordance with their shares of inheritance.” There may be some among grandchildren who inherit. This is our view; it is also the apparent meaning of the words of our companions, and it was given precedence in al-Furoo‘. End quote . 

What you should do is return this gift, and explain to your father that it may cause some resentment in your brothers’ hearts, so there is no need for that. 

This idea is also supported by the fact that this gift will in fact be for you, and you will be the one to benefit from it, not the small child; hence your father has favoured you over your brothers by giving it to you, and that is not permissible. 

And Allah knows best.