Wow, I have never in my life heard of such a thing
What was the reasoning behind him NOT being able to neuter him? Did he say? I have just never heard of this
I would possibly look into finding a new vet
If he said he "can't" neuter him, I would personally be a little nervous about taking him back in 6 months. I have just never heard of a vet saying that they can't neuter a dog.