Just get him in. It is great for their coordination, improve muscle strength, memory and teamwork skills. The younger the better and the more likely to be unbiased about gendered activities. If he likes it, he is more likely to continue. Otherwise you could try hip hop, it is not that girly. When I was a teenager from time to time we would get the american football players of our school join our jazz and pilates sessions for a whole year to help them loosen a bit, be more flexible and improve coordination.
I went to my daughters recital and there were a few boys in the classes for modern and tap and one boy in ballet. They loved it and enjoyed themselves and it was so much fun. They also got the biggest hugs from their mum at the end. Trying new things is scary but those boys were so proud and I’d be proud if my boy was in a dance class. Listen to your heart and give it a go, I say!
My son is currently in hip hop class and was in ballet and tumbling. I am shocked your husband would be against it! All pro athletes have taken some form of ballet. It helps with lower body strength and stretching and flexibility. Ballet class helped my son with his balance in flag football and tumbling class was mostly stretching that he also does in karate class! His strength in his legs and his ability to kick higher improved a lot from these dance classes.
I go to baby ballet with my daughter and wear my newborn son. There is a boy in the class and there are a few dads who come with their kids. Do you have babyballet in your area? They do a discounted 3 lesson taster and if you like it you can sign up. Theres a boy and a girl mascot too.