Until recently we had two full dalmation males together with no problems the younger one was introduced aged 18 months when the elder was 7.
Sadly recently we lost older boy leaving the younger one on his own now aged 7.
We have been thinking about introducing a puppy but have recently been asked having done so before  to re-home another male dog aged 11 months instead of getting a puppy.
The new dog is fantastic  and they seem an ideal mix except he has been castrated and on first meeting our dog treated him as a female and bothered him to the point that the only option was to keep our dog on a lead and away from the younger dog.
Is there anything that we can do, will this obsession fade with future contact and familiarity or should we just accept that this puppy will have to be re-homed elsewhere?
Can anyone help, as we really would love to accept this new dog.

Feminisation syndrome is one of the side effects of castration. You can treat with male hormone but they in turn reduce the dogs immunity which leads to other problems, so unless the problem gets too bad I would just put up with it, but if it starts to cause trouble then you can either treat the castrated one or have the other done too. The HRT is expensive and as I have said can cause other problems.