If you only want to marry and not have him stay in the US or work in the US, he does not need to have a green card to marry.  One of you needs to have satisfied the residency requirements for the place where you intend to marry.  If, however, he will be staying in the US after marriage for an extended period of time and will need to work, etc. then you will need to petition him for a green card after the marriage and in that case, you should not marry within 90 days of his entry but wait until it has been about 90 days.