Is he a freelancer and you a permanent employee?
In my market it's quite normal for dev jobs to be ±5k perm and ±10k freelance.
same exact job and skills
same perm role
He asked for more. Simple as that. So you start working on your resume and on your next interview you set salary bar higher.
When you have another chance to negotiate, don't forget about non-monetary forms of compensation. More vacation time, training budget, perks. Money isn't the only part, it's just one part of the whole picture.