Mimimi and other friends, please help on my son's math

My son is 15(it's his birthday today). He got a D+ in his Honor's Geometry class,the lowest grade in his class. He's a 9th grader. His teacher said he is smart but it takes him longer to process and apply the information and they are moving fast in class. She suggests getting him help if we want to keep him in the honor's class. My son said she doesn't explain in class and wants to change to a different class. I don't think I should blame the teacher because other kids have higher grades. He needs to put more effort. We started Kumon for him 3 months ago and he didn't like it but we think it's necessary. He was tested at a very low grade at Kumon Center. My hu*****and helps him sometimes but he is too busy and he isn't patient enough. Me too. I get mad when I explain to him and he is impatient or appears to be uncaring. So I don't know whether I should find him a tutor. He said he'll try the school's tutor first.