So if I get my boyfriend a fiancé visa and he comes can he stay after we get married for than 90 days?

Yes that's correct. You must marry within 90 days after he enters the US on his fiancé visa, and then he can immediately adjust status to get his green card. It takes about 9 months to get his fiancé visa after the case is filed, and then after he enters the US, it takes another 9 to 10 months (after filing his adjustment of status) until his green card interview which you both must attend. My fee to represent you from start to finish is $1,000.  (Made payable in 2 installments).