If he hasn't responded, it could be because maybe he doesn't wish to sell the domain or maybe he is inactive with the domain. Another possibility could be that he is inactive on the email id he has registered the domain with. If he doesn't wish to resell it, there's nothing we can possibly do. There is however some hope that if he is inactive with the domain, by the time his purchase expires, if he doesn't renew his registration, it will be made available for purchase online and then you can purchase it.
Why don't you try a similar domain name? or innovate the domain name so that you may find a possibly vacant domain.